Sounds familiar to me. I've had the M500 for 4 days and my first impression was:
Very little bass, near no sub bass, very cristalin smooth high and lush voices BUT a snappy peak in the higher mids somewhere making them sound very fast, snappy, yet sharp in a not so pleasant way to my ears. I was listening with a Tera player, kinda notorious for being hard to match with low impedance headphones such as these M500. A couple of tera owners recommended them so I gave them a try.
I thought these were designed to be portable so I assumed, maybe they'll sound better with iphone... Nop! A little better but still way off to my ears.
Finally yesterday, I decided to try them on a HM801 DAP and WOW! The bass and sub bass were back in the picture, the undesirable peak was gone and everything sounded a lot more balanced and neutral. My M500 still need to burn in at least another 100 hrs but still. I can now hear the true potential of these cans and I know they will sound pretty good.
I played with the EQ a little to add a tad of bass and they can deliver plenty of bass and sub bass granted they are properly matched and maybe given a little boost.
They still have much speed and attack (reminiscent of LCD2s that I've had in the past) but also matched with an overall liquidity that I find quite stunning. These phones are no light weight, I think it is preferable to match them with warmer sources that also offer EQ (like HM801) or amps (don't know much about desktop/portable amps).
If I didn't have the HM801, I'd say these headphones signature really suck. But because I have heard them better matched, I can say they sound pretty amazing already (not even being fully burned in!)
My two cents!
thanks for this info...good matching is still the key to any can soundg its best. 801 isnt cheap...anything more down to earth$$? :P