I have the 627 and the 627x. I find myself preferring the 627x because of the massive soundstage and equality of presentation across lows mids and highs. I find the 627 to have really great mids, as previously mentioned by many. They are super smooth with high resolution and a more upfront presentation than on the 627x. There are many who pref the 627 over the 627x for this reason. However, doing an A/B test between the two, I don't think the 627x has worse mids, it's just that the presentation is not as midcentric as the 627.
The 627 manages to sound more realistic because of the upfront nature of the mids: as they're recessed on the gen2 the immersion isn't as intimate. But the 627x has that soundstage, so it's a different kind of immersion being achieved, which i prefer.
Cesar upgraded the potentiometer in the gen 2, and it really does sound like a big brother amp to the gen 1. After A/Bing for about 8 hours, I consistently end up switching back to the 627x for extended sessions.
system:
MOG 320kbps streaming service > ifi mercury usb > ifi iUSB > ifi gemini > ifi iPurifier > ODAC >Audioquest Victoria 3.5 to RCA > ifi iTube > audioquest victoria 3.5 to RCA > 627x > headphone lounge silver occ custom cables on shure se846's.