I have! I wrote a tiny bit here: https://www.reddit.com/r/headphones/comments/nfewef/sennheiser_hd660s_impressions_versus_older_hd600/

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In general I've found every HD600 variant I've heard to have a little too much 1kHz elevation which makes them sound just at the threshold of shouty to me, so I found I never wanted to use it so I've gotten rid of it every time I've owned one because I already have way too many headphones for someone that uses speakers 95% of the time. The HD650 usually has a little bit more elevation in the mid bass and less elevation at 1kHz so it sounds more relaxing. The exceptions are ones with paper baffles, which are rare enough that you really have to look for one to end up with one of those so you can assume that if you buy an HD650, you'll get a more relaxing sound than the HD600. They have a little more damping behind the driver than the HD600 so they do suffer a little in dynamic contrast though.

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I'm in the "scale" team, as you probably could guess by the fact that I actually took time to list out my chain used for this review lol. But I certainly can't recommend most people buy a mid-priced headphone and also buy a mid-priced chain unless they have heard plenty of other headphones and find that they still like that same headphone more than the others. After over a decade into this hobby and having experience with most modern flagships, I still can't get over just how well the Senns do midrange and I'm willing to give up some resolution for that midrange, since most flagships can't live up to them in that one particular area. A lot of the music I listen to focuses on vocals though, so the things I index for may not match yours.

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The best recommendation I can give though is to try everything you can off a cheap source to figure out your preferences. Then if you want to get into the nervosa of gear scaling, buy something that has good resale so if you find you don't hear a difference after a couple weeks, you won't be stuck with it. I've certainly sold a lot of gear because I found it either didn't make a difference or it sounded worse to me.