I've been drooling to get a decent pair of headphones for at least a year.
A little while ago I had chance to go to a city that had some audio shops and compare HD600 and HD650, unfortunately it was unamped using my Desire Z. In this manner HD600 sounded a lot clearer and a lot like what I was expecting, compared to the really muffled and uninspiring HD650. I was in belief that HD650 is very much the same as HD600 with just a touch more bass. I like more laid back sound, but not at that cost in clarity.
This left me wondering if the HD650 suffers more from bad/non-existent amplifying, or are they just that different. (or is it bass heavy headphones in general that need better amplifier)
P.S. And oh god it was a BIG mistake to try out HD800
All of the HD's you tried really need some amping and will then sound terrific. In any case the HD650's have a tad more bass and a tad less treble which is why I ended up with the HD600 as it was more neutral. Yes trying the HD800 was a wallet inducing stress factor. I have a new ear toy on the way on the FedEx truck but not an HD800.