AD900x when in the mood still my favorite headphones. They are more fun sounding than the MSR7. The mids are more recessed I think on the MSR7? AD900x mids are fuller, and prominent, gives it a heavier sound. AD900x are warmer than MSR7. Definitely a more airy sound, the bass isn't as tight though, bass is better on MSR7. The highs are more "fun" but the highs are more detailed on the MSR7. But for a fun headphone, AD900x definitely probably at the top, but K712 Pro is more resolving and detailed than any of them. The treble on AD900x I think is "fuller?" and MSR7 bit thinner/tinny at times. But for a closed that isolates this well, MSR7 is beyond my expectations. I found headphones with bass like that often have subdued mids and too much emphasis on treble and for me MSR7 mids is best I've heard for closed.
There is something magical about AD900x. I know the sound is more congested, not as detailed as K712 or MSR7, maybe not has high res sound, but they are simply more fun to listen to when I'm listening to EDM, rock. AD900x has IMPACT, on both bass and treble, and mids are sweet. The music just slams and impactful unlike the other headphones. I can't put it into words since it's more emotional than sound I think, the impact is what makes AD900x special for me. And the treble is harsh enough that if I blast the music, doesn't hurt and it's not shrill. Unlike the others.
While I may rank the K712 and MSR7 above AD900x in overall sound evaluation, if I could only keep one, it would be AD900x. May not make sense to others, it's not as great as others, but it just has more impact and fun.
The problem with AD900x is while it has the airiest sound of my headphones, sound can be congested, K712 Pro is the opposite, the separation is best I've heard (I don't spend more than $450 on headphones). I'd say MSR7 has better separation than AD900x.
MSR7 has more similarities in mids/treble to K712 to me, but the bass on AD900x. It's just a much better headphone than K551 for me. K551 for me would cause ear ringing from that harsh treble, it's an enormous amount of treble, and mids just not up to snuff. I listen to MSR7 at lower volume than any of the headphones, K551 I listen to loudest because the mids/bass just aren't enough, but then treble probably ear damaging (treble damages your ears more than bass does).
It's hard to compare AT to AT and then compare AT closed to AT open. MSR7 is just surprisingly good sounding for a much smaller, isolating headphone I'm used to. It's more detailed than any Sennheiser I've heard (up to HD650/Momentum), and Shure SRH840.
Bottom line though, sitting at home, I'm going to be grabbing AD900x when I just want to blast music, K712 Pro when I want the separation and details. MSR7 is the portable, open air > closed.
My review/impression are probably awful. But there it is.