I have owned all of the above hp's but haven't heard the o2. Sorry to make you repost the hp's you will use.
I can say that the vali will do great for the he-400 and pretty good for the 650. the dt990, even though it is somewhat bright I would still use solid state to help its bass out. I can't picture the smooth bass of the vali going well with the airy bass of the dt990 but it may help its timbres or music tones out. I would picture the he-400 doing great with the vali.
the o2 was built using the hd650 as a reference so you cant go wrong there. I have used the hd650 with other tubes though and it was nice despite its already smooth nature. I don't think you can lose either way. But if the o2 is as analytical as they say then the he-400 and the o2 may make an unpleasant listen. The he-400 imo can use some tube action for sure... mind you the vali isn't as tube sounding as most tube amps so its like a safe bet.
Also if you ask anyone else on this thread they will most likely say the vali because it is comparable to other Top Of The Line amps in quality....so they say.. im too poor for those $1k amps so I wont know anytime soon.