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!
