I'm in the camp that would have voted for the HD650 had they been a choice. I voted for the DT880 which is a close second. From what I've heard so far, the HF-1's are also a good candidate for a liquid, smooth, unoffensive midrange. I've only listened to my HF-1's for about 5 hours, but they're quite good right out of the box. Sade, Dylan, Neil Young, all pretty much what I would have expected in terms of vocal smoothness (or lack thereof in the case of Dylan).
The RS-1's, SA5000's Stax 404's all have a much more 'prominant' midrange (i.e., you take note of it right away because of its forwardness) than the HD650, DT880 or HF-1, but I guess it depends on what you mean by liquid. To me, liquid implies that the midrange gets out of the way and just blends in with the highs and lows without necessarily calling undue attention to itself. If you're looking for 'smooth' vocals (as opposed to headphones with a more aggressive, foot tapping PRaT), then the HD650/Zu would be my first pick. Oh wait, the Zu will push it past $300. But I'd still pick the HD650 and upgrade the cable at a later time.
If your definition of 'liquid' is otherwise and what you're really looking for is ultra detailed and resolving (i.e, microdynamics), then the SA5000's will be tough to beat, other than perhaps by the Stax 404's. The RS-1's are also very resolving and the HD650's are no slouch either, but this is where the DT880's and HF-1's start to get left behind (that is, they have smooth, liquid midranges, but are not nearly as detailed and resolving). This is why I think the HD650's are the best overall blend, and they've got a phonomenal bass response to boot!
Don't know much about the 501's or A900's other than brief meet impressions on a couple of occasions, but neither of them has really grabbed me. Maybe it's the lack of exposure to them, so I can't really say. The HD600's would fit in the smooth, liquid midrange category, but (at least my take on it) is that they're a little lack luster (err, laid back) in other ways. Yet, if what you're after is an "easy listening" type of headphone that is totally inoffensive, the HD600's are probably the most overlooked headphones on the planet for this!
(Never would I thought I'd describe the HD600's as 'overlooked' dating back about 3 years ago, but they really have gotten lost in the shuffle of late).
