This is also solid advice. Again, I’ve only spent a limited time with the RAD-0, but those few minutes left me going “WOW.”
Still, I think I’d take Caldera.
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Thank you! The comparison to the Arya, in particular, is very helpful and leads me to believe that I would like the Caldera.
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Caldera. Not much to add to what others have said, but Caldera is just on another level vs all the other ZMFs. The Atrium has a wider soundstage and more forward mids, but Caldera wins on basically everything else. I actually still prefer the Verite Open over the Atrium slightly but really all 3 are worthy of TOTL status among the ZMF open backs.
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Thank you so much for your input. You’ve given me a lot to chew on.
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I personally prefer the Atrium between the ZMFs in play, and it’s mostly based on timbre, and a bit on sound stage. I just find the Caldera’s timbre to not sound quite right or “natural” to me - but full disclosure - I have found that to be the case with virtually every planar I’ve tried. Many many many audio enthusiasts love planars, and planars can do things dynamic headphones cannot, so please only take that as my own personal leaning rather than a suggestion of an objective truth. If you have found the timbre of planars satisfying, I don’t suspect the Caldera will be any less so. As for stage, there is something magical about the Atrium, and it’s not that the Caldera has poor staging - it’s quite good - but the expansiveness and depth of the Atrium is just super unique.
I am also going to throw a wrench into this whole mix and suggest you may want to explore the Utopia - and I promise I’m not saying that because it’s my favorite headphone.
The Caldera wins all day for sub-bass extension, and it’s plenty punchy. The Utopia is hard to beat for punch nearly everywhere except super deep sub-bass. The Atrium isn’t a slouch either, but if you want to go subterranean, that’s Caldera territory. None of these are going to disappoint, I don’t think, for general punch, so it’s mostly a question of how important the deepest depths are.
The Caldera is more resolving than the Atrium, but the Utopia is more resolving than the ZMFs, and noticeably so.
This is really hard to pin down and gets even more subjective. The Atrium is less resolving than the other two, but I find it resolving enough not to be boring. Also, the Atrium has a very natural, if a touch warm-leaning timbre that is super satisfying and organic, and is the most “fatigue proof” of the three, in my view. I already noted my feelings on the Caldera timbre, but again, I thought it was in line with other planars. That said, I definitely never found the Caldera fatiguing - I think it’s super pleasant. With the correct chain, I find the Utopia sounds the most “right” on average, across all instruments and sounds, in terms of timbre, and won’t be fatiguing if you build the right chain for it. Beware - the Utopia is a microscope for what you’re feeding it, so you can absolutely cause it to be fatiguing if the sources are such. The ZMFs will both take the edge off a bit, so it’s less risky.
The last thing I’ll note is that, if you ever want to get into tubes (which I’m not necessarily suggesting - they’re a beautiful pain in the butt), the Caldera generally isn’t great off of tube amps. Not terrible, but not great. The Utopia and Atrium are just delicious on the right tubes.
Having read through other folk’s suggestions, I pretty much agree on all of their notes, so, absent an ability to try the headphones out, I think you have a lot of really reliable feedback to go off of. That doesn’t mean the decision becomes easier, but in the end, the Caldera and Atrium (and Utopia!) are all masterpieces in their own right, and you will likely love any of them. I know I would!
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Not sure I agree there, although I can only comment on my own tube amps. I think the Caldera has great synergy with the Nautilus (where on Zach’s advice, I plugged it into the 100ohm tap to give it a bit more body).
If I want a bit more color, I was surprised at how much I like it on the MZ3.
Edit: I’m obviously not discounting your own experience but I’m curious what you based that comment on. I wonder if it boils down to different preferences or music, or it works better on some tube amps than others.
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That’s great to hear!
I heard the Caldera on my Stellaris as well as a Woo WA5-LE (supposedly good w/ planars), and I didn’t think it benefitted from either. I’d say it was “meh” on the Stellaris - not terrible, but I would never prefer it from the Stellaris vs solid state (straight out of the DAVE). The Caldera sounded better out of the WA5-LE than on the Stellaris, but even then, I don’t think I’d ever prefer the WA5 to solid state, even for “flavor”. I just didn’t think that either amp added anything desirable vs a good solid state option. Put another way, they sounded like slightly limper solid-state amps, but didn’t present the “tube magic” as they do for other headphones I’ve tried.
This is in stark contrast to headphones like the Utopia or Atrium, which I think just straight up sound better out of tubes - more three dimensional and lifelike. I just didn’t get that with the Caldera, at all.
No offense taken at all! I think it’s super helpful for @Kennyboy to have additional input from other amps with other preferences. I was mostly surprised that I didn’t think much of the Caldera on the WA5-LE, given that amp’s reputation for handling planars, but if you like the Caldera on the Nautilus and MZ3, that’s awesome. 
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One of my favorite headphones I heard at CanJam!! @zach915m crushed it with these planar headphones!
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This is one I’d like to hear myself. So many opinions on these. Seems kinda polarizing. Same with the Atrium.
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I like them! I also highly enjoy planar headphones and with ZMF magic I was really impressed.
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Getting back to @Kennyboy with his conundrum of Atrium vs Caldera, I think the best solution is to get both! 
In all seriousness, I doubt you’d be disappointed with either. They would simply be rather different, depending on which route you want to take it. I think either would be quite satisfying in different ways.
I’ve not heard the Caldera, but I do have 3 ZMFs: Atrium, Verite, and Aeolus. Each one is different, with its own emphasis but having a common “house” sound to them.
The Atrium has phenomenal midrange and excellent bass and slam, with a very smooth treble. It’s not as “sparkly” and resolving as Verite in the treble, but it is such a blast, you can and will listen for hours with a big smile on your face. The Atrium has a soundstage unlike most anything else I’ve heard, it was a bit startling at first in how 3 dimensional it is.
@pennstac mentioned the Rosson RAD-0, it kind of ties Atrium as perhaps my favorite listen. It has a similar tuning and timbre in many ways, but with the slam and resolution of a planar. Soundstage is good but not on same level as Atrium.
My 2 cents worth… 
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Without having heard either one, I have developed a rather fatalistic view on the question of which one would I prefer, the Atrium or the Caldera. Such a difficult and nuanced decision. Or prediction.
I finally realized that I was struggling with the wrong question entirely. The real question is: which one am I going to buy FIRST.
So that means the “risk of being wrong” is merely a timing issue.
YMMV.
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Can’t speak on the Caldera but the Atrium is a great sounding headphone. The overall tonality is just so pleasing and the biocell driver has quite a natural sound especially with drums. Definitely not lacking in punch and is in fact one of the best bass response I’ve heard. Do consider the Utopia as well if you are looking for a more resolving and more neutral sounding headphone.
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Zach has a ton of tube amps, which he uses to test new products, which is why I think his headphones generally sound good with tubes. But that logic should apply to the Caldera too.
If CanJam New York is anything like Chicago last year, and ZMF is going to have a room, then I’d suggest trying the Caldera on whatever amps they bring to NY, to see if you change your mind. Worst case, you come to the conclusion it’s not for you.
@Kennyboy - do you live close to any of the CanJam locations? That would give you the best opportunity to try different headphones, and decide what you want - it might not be a ZMF, e.g. you might fall in love with the Utopia.
As you’ve seen from everyone’s responses, one isn’t better than the other, they’re just different. I liked the Atrium and Caldera equally, but I bought the Caldera because I didn’t have anything else that sounded like it, so it was a good addition to my collection.
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I wouldn’t necessarily make that assumption. Planar magnetic drivers are a different beast from dynamic drivers, and it’s fairly common that planars are tougher to pair with tubes than dynamic drivers - and it’s also fairly common, including outside of ZMF, that dynamic driver headphones pair beautifully with tubes.
That’s not to say Zach hasn’t tested the Caldera with tubes, and it’s not to say that they sound bad on all tubes, but I also don’t think it’s that big a leap to end up with a headphone that is generally less tube-friendly when using a generally less tube-friendly driver type - emphasis on generally.
Regardless, this is all subjective, of course, so if other folks like the Caldera with tubes, then awesome! I just wanted to add my two cents that, as a tube lover, I didn’t think it paired well with two highly regarded tube amps, including one that is often marketed as planar-friendly (the Woo WA5).
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You’re so right about planars & dynamics being different in their synergy with different types of amps.
I currently own 2 planars (ZMF Ori; Final D8000), and both actually sound well above average on my modest but terrific-sounding transformer-coupled tube amp, the Icon Audio HP8. And each one sounds OK on my modest OTL, the Woo WA3. But they only really come alive with powerful SS amps, particularly the Violectric V281 and Monolith LGX.
I had a loaner Caldera here but didn’t have the chance to do much listening with it. I did briefly try it on the HP8 and V281, and got exactly what happened with the other 2 planars: excellent sound on the HP8, but even better sound on the V281.
What I don’t have here are any of the big, heavy, costly, multi-tube OTL or TC-tube amps that some users have. It may well be that planar headphones of all kinds sound even better on big tube amps. I just don’t know…
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I live on the Virginia Peninsula in southeast VA. Not exactly near any Canjam locations. So I rely on reviews and impressions of others. The Utopia is on my shortlist and has the advantage that used 2020 models can be had at a nice discount. Since I like my Clear, I suspect that I would like the Utopia. If I buy a Utopia, I guess that I will sell my Clear OGs. But when I learned about the Caldera and the Atrium, they sounded equally as desirable and as worth consideration as the Utopia.
One reason that it is hard for me to decide on a flagship headphone is that I don’t have experience with an exceptionally high resolving headphone, so I don’t know what I am missing.
I appreciate the input from yourself and others in this community. Some experts have no patience for relative laypersons like me. I find only helpful comments here.
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Don’t worry about CanJam – go to Capital Audiofest near DC in the fall. Plenty of headphones to try. But, not many ZMFs.
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Haven’t heard the Caldera yet but from what I hear it’s a bit of a chameleon and tends to change with amps quite a bit. I have heard of a lack of synergy on Stellaris and Studio B from several people and since those are/will be my main amps, I’m likely not going to end up with one 
I do have both OG Utopia and Atrium and they are quite different and I love them both. Unless you care a lot about the wood type, stock used Atrium can be had for under $2k easily (same for OG Utopia).
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I think we’re violently agreeing.
I just meant that Zach has at least one tube amp that can drive planars well, and would presumably have used that amp/those amps for part of his testing - this is conjecture of course. If he brings that amp/those amps to CanJam NY, you could see if it’s just a synergy thing with the Woo. I wasn’t inferring that all ZMFs sound good on all tube amps. And in the same way, if the Caldera doesn’t sound great (subjectively) on your tube amps, I don’t think you’re inferring that it doesn’t sound great on all tube amps.
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