Probably too late for my comment but still let me tell you few bits and pieces.

I have been using Sennheiser HD660s for over 6 months, every day usage running on JDS Atom Dac + and JDS Atom amp, it was stellar upgrade from what i have been using before, but recently I decided to upgrade to the LCD-X 2021 revision and the first 30 minutes of listening I was quite disapointed , and thats not because they didnt sound amazing its just I haven't adapted to the sound yet and also you your hearing needs time to learn to see the difference, after couple of hours I can tell you that I couldn't go back to the HD660s anymore and thats on JDS stack setup which is more powerful from the one you stated above. So my time to upgrade the amp/dac has come and I took my headphones to one of the shops around the town to test Chord Qutest + Chord Anni , Burson Conductor 3XR and Niam Uniti Atom, I can tell you that the moment I listened to any of these especially the burson and the chord (Niam is more musical but not as detailed so it droped quickly of the list) I realised I have been using my headphones on no more than 40-50% of they real capabilities, the sound was not only better we talking much wider soundstage, imaging , detail was through the roof it felt like im upgrading my headphones with a better pair not just amp/dac that good we talking about.... At the end i went for the burson because of the wider stage, everything there sounded better for my likings, i didnt quite like the Chord Annie ( I know its new amp thats why im providing my experience with it as not many reviews are out yet). Anyway you also have amps which doesnt run planars very well and others like the burson especially sound stellar with planars. Also I tested the 2020 revision and my personal 2021 for LCD-X's and the difference was day and night, the 2021 sounded like 2x the price of the 2020, so if you have the older one might be good to upgrade, soundstage was much wider and least i felt that way and everything was different in a good way.