x-posted
humanflyz I can agree with most of what you said about the HD650 (I've only briefly heard the W5000 on a few occasions so no comments about that).
I know when I first registered on Headfi 2 years ago the HD650 was a really popular headphone for having excellent soundstage. I heard them on and off and wasn't really "blown away", which is fine since most headphones that did that to me were usually fatiguing in the long run.
I've had the HD650 for a while now (bought them after the K1000 as a bassy alternative). And the 3 biggest flaws that I see are:
1) the soundstage is much smaller than most people make it out to be (maybe they haven't heard anything better or they switched from a Grado ). Even then the Grado RS series have a very nice headstage (to me it's very close to the HD650), but a smaller soundstage.
2) if I switch from listening to the K1000 to the HD650 I immediately notice the instruments are well placed left and right but there isn't much height, or anything between the left and right and highest point; it's pretty much empty. This effect is most obvious for most music except for some studio rock music. This is fine for the "focus area" of the intended recording (usually vocals, which the artist probably wants you to focus on) which is usually placed directly in front of your ear but the rest of the imaging just isn't concise to my ears.
3) the slowness you're describing. This is probably the most difficult to describe, but more than likely I'm perceiving it as lack of PRaT (pace, rhythm and timing). One of the few pieces of audiophile jargon I feel I've come to grips with since this ultimately determines how fun something sounds to me.
I've just said a lot of bad stuff about the HD650, but I haven't mentioned any of its strengths which it has plenty of. I'm going to hold onto these for a long time, and I do hope that going balanced will solve at least 1 of the 3 points I put up above. Hope this helps someone, I really shouldn't be using my break time with these lengthy replies