IMO the 58X have more “bite”, especially in the bottom end. IDK if that term makes sense to anyone familiar with both cans; but it’s what came to mind for me.
I have plenty of ear time with the 58X600/650 and sometimes miss the HD600 because of how it does certain types of music so very well (eg baroque).
It might also be due to some nostalgia at times, with them being my first decent headphone since the AKG340 way back when.
Well, I’ve been spending some good time with the HD58X lately and came to a realization. In stock form, it’s a bit thick on the low end and a bit sharp up top. I didn’t remember being bothered by this when I first tried it and I’ve since messed around with EQ and mods a little to try and address that. Well, I hadn’t used crossfeed for a while and on a whim switched it on (in Neutron App) and boom, the HD58X sounded as great as I remembered from the honeymoon period. It turns out that at the time I had been using crossfeed extensively but had started to move away from it while using other cans like the DT 1990 because it thinned out the bass too much and dulled the treble.
Well, duh! That’s just what I think the HD58X needs, and I get better soundstage and imaging in the bargain. What’s not to like?
I have both. The 6xx didn’t come into its own until paired with the Lyr3. Unless adequately powered, the 6xx may not sound better or more detailed than the 580. Sennheiser underestimates the amount of power needed to get the most out of the 650/6xx. My Dragonfly Black won’t do it, nor does the Dragonfly Cobalt, nor even my iFi xDSD, all of which are fine with the 580.
The 650/6XX power requirements you mention are why I believe they sound so much better balanced.
I have a modest but very clean balanced amp with a somewhat outdated balanced DAC.
But the HD650 still sound better on that set up than on many lower power SE setups costing as much or even more.
I imagine a newer (and necessarily more expensive) high powered SE amp would close the gap considerably. And I also imagine it would eliminate the gap and squash my setup entirely, if using something like a Phonitor X and a high end balanced DAC.
Still, I’d love to have an opportunity to compare the SQ difference before spending 7->10X the $ for the improvement. 
The HD58X on the other hand IMO doesn’t benefit as much from the extra power. And I can see why crossfeed might take some of the “bite” I mentioned previously out of the top end. I don’t find the 58X troublesome on the low end; but it’s certainly not as polite as the 650.
Sometime, when I finish procrastinating, I’ll do a review of the Lyr3. I haven’t had a balanced amp, but have run dual-mono mode on the iFi xDSD. Before I fried it somehow, I used a modern Headroom Standard amp. Neither of those has the oomph of the Lyr3. Anyway, it clearly makes a difference with the 6xx, at any volume and throughout the frequency range. The price on the Lyr3 was not too bad - I didn’t get any of the add-in boards. All I needed was the amp.
Is it true these guys basically use something like HD 599 drivers? I never heard them before but it seems to extend better than a lot of other 600 series headphones but maybe on par with 599?
Welcome, Rena-
I’ve always heard that they are the HD-580.
I don’t think there’s any drivers about which people have speculated more than the HD58X! It does look like the sub-bass extension is indeed similar to the HD599 - https://www.rtings.com/headphones/1-2/graph#891/2031/408
However, the distortion characteristics seem wildly different, so I kind of doubt it’s the same driver.
https://www.rtings.com/headphones/1-2/graph#891/2085/408
I know that the “HD58X is retuned HD660S” hypothesis is unpopular these days, but looking at the distortion characteristics they sure do seem related:
@prfallon69 @SenyorC @pennstac
Thank you for the welcomes!
@pwjazz
I always thought they were lower grade drivers than the standard HD 580/6x0. 
Hmm could be though but that would be a pretty good deal if it came close to it.
HD58x on sale right now on Drop. $135!!
I took off my HD58X’s pads for cleaning a couple of days ago and noticed that the foam covering the drivers on the inside is surprisingly dense and thick. I’ve heard some people report positive improvements from removing foam, though I had always assumed they were talking about the rear foam which in my experience doesn’t change the sound. So today I removed the front foam and it does indeed transform the HD58X, bringing the upper mids and treble up, and the mid and upper bass down a tad, bringing the response more in line with something like an HD600 (except with better sub-bass extension).
Hello clarity, hello detail! Keeping the foam out isn’t a permanent option because it’s there to keep dust out of the driver. Time to find something acoustically transparent that I can use as a dust shield. Suggestions welcome!
Oh wow! I decided to use some sheer curtain fabric to protect my driver from hair and such. It’s not completely acoustically transparent, but it actually brings the lower mids and bass back up to stock and leaves the upper mids and treble open and airy (with less of a peak at 8 KHz). This sounds frickin’ amazing and looks slick too!
I see a business opportunity here. 
Just a thanks to Z and this thread for leading me to the end of my audio quest. P20 to HD 58X is it for me I won’t need to search any further. Been looking for something that equals the JBL’s and Marantz combo I had 40 years ago and this is better!! Will be getting the upgraded power supply and tubes in the next few weeks but can’t imagine it gets any better. Thanks again I’m out.
Hey, this is only the tip of the iceberg. 
58X was my first one (from drop.com). Then I bought 6XX, then the …
Good timing, this evening I took the 58X I started hearing some songs. Then Creep by Radiohead started playing. When the crunched guitars kicked in I had the “HOLY CRAP” moment. Yeah, that feeling… 
This is the reason why we buy those cans.
Good choice my friend. Welcome to the community!
Welcome and nice buy. I bought a Senn HD-580 Jubilee back when it first came out, and I found I couldn’t part with it, even after getting the 6xx and some other more expensive headphones. It’s still in rotation, and still very much enjoyed.
I’ve posted a video detailing the curtain mod.
@antdroid asked how this compares to an HD600. Here are some measurements (E.A.R.S. HEQ compensation, psychoacoustic smoothing). Sadly I don’t have the HD600 on hand anymore to do listening comparisons.
