The HD555 are very popular due to their price point. I think you should expect them to be very very comfortable for one. I suspect they will have a hump in the midbass region, a good midrange and treble that is a bit more laid back compared to your M3's.

These might be a headphone which you can wear for hours without experiencing any listener fatigue. $84 seems like quite a good price.

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne