While not directed at me I still have my HD650, HD6XX and HD58X, previously owned a HD600 as well and have borrowed a HD660s from a good friend on a few occasions.
HD650/6XX: pretty much the same but for some the 6XX is just a touch brighter and IMO really needs a OTL amp to shine.
HD600: what most would consider the “standard” as a measuring stick but for me they have a small peak in the 3.5-5khz range that bothered me at times and for me they are a touch lite on the bass for most of the genres that I listen to.
HD58X: closer to the HD650 than the HD600 and not as amp picky.
HD660s: If you don’t want to bother with amps/DACs then this is an easy pick since it sounds quite good from almost anything IMO. Sound wise its closer to the HD650 to me than to the HD600 due to the nicer bass and no peak in the upper mids/lower treble like the HD600.
To your question, HD600 is closer to HD650/6XX to me since the HD58X has more bass than the HD650/6XX on my gear but the biggest issue to me is that they are also a bit grainy in the highs compared to the others in the 6 series.
Leave it to Tyll to have some comparisons done years ago.
Amen.
May he be fine in his van and get everywhere enough power, to listen to his headphones
@ProfFalkin, @metal571, HD6XX(650) or HD660S…? 
Hd6xx all day long and twice on Sundays.
How do they compare?
Dunno. Haven’t heard the 660 beyond showroom floors, but my impressions were meh. Partially because the conditions were crap, and partially because I’m beyond satisfied with the 650.
Hd650 is the gateway drug to ZMF.
True story.
Are you saying ZMF is similar in sound signature to the HD650?
I like 660S the best with no EQ but the timbre of the treble isn’t quite up to 600/650 par. 6XX with the upper mids pulled down is my favorite though. Or the 650. I only have a 650 because I like the way it looks more. Same exact headphone - I compared it side by side with 6XX
Interesting. 
Looks like I’ll have to order a pair from Drop.
If I have further questions I’ll move the conversation over here:
I think I’ve cluttered this thread enough. 
Thanks!
May upgrade to some better noise cancelling headphones if something much better than the Sony 1000MX3 come out.
Side-graded my Topping DX7 to a SMSL-M500 (I’ll call it a side grade as I don’t know that they are considered that far apart)
I need to upgrade my attitude (according to some) but I’m going to hold off on that until the price is right.
Lastly, probably will update/change around the setup a little bit. Desk feels a bit messy/sub-optimal as I’ve added a fair bit of stuff over the last 18 months so want to get it less cluttered and pretty like.
Same here (regarding organizing).
Every time I order something and the package arrives, my partner asks me “where are you going to put that?” 
I think I just need a bigger home.
Yes, but that’s not to say they sound the same. I’ve found that those who like the mids of the hd650 will usually gush over how Zach’s house sound jives with their preferences.
Edit: For example… This was just posted on HF.
I’ve spent over a decade listening to and enjoying the HD650/600 sound signature, and these just seem like a better version of what I wish the Sennheisers always sounded like.
I definitely need a bigger place, though my condo does limit the level of hoarder I can become. 
I’m going to give my iPad Pro as my primary player a try; it really works well as a player when I travel (so far), passes MQA like a champ without much hassle.
@ProfFalkin This is exciting for me to hear, a lot of people said the same thing about the Elear from Focal but I don’t really think they are that similar in sound, I do enjoy the Elear don’t get me wrong but they sound fairly different to me. So naturally as a lover of the 650/6XX sound this excites me to hear. What set from ZMF do you think sounds the most like the HD650? How does the sound stage, and detail retrieval differ in your opinion?
I use my iPad Pro daily for listening!
ZMF Aeolus is my favorite in the vein of the HD650/6XX…but exponentially better to my ears!
I think that’s going to be my save up for headphone Christmas 2020!!!
I don’t think the HD6## series really sound like the Focals I’ve heard either.
Probably Aeolus or Atticus. Less the Atticus, as it kind of goes “full retard” in the mids. Let me sit and listen to them and gather my thoughts so I can be specific in my reply.
thanks buddy!