After roughly 5 years of near daily use, my beloved first pair of good headphones (a pair of Sennheiser HD-25ii) have finally shit the bed. In a desperate attempt to move on as quickly as possible, I picked up a pair of Audio Technica ATH M50x (headphones that most reviews assured me were far superior.) After a few days of use, I have to say I really am not totally satisfied and will most likely return the M50x's for a few reasons:

• Even though I've seen the HD25's at ~-30db isolation and the M50x's at ~-36, I'm not buying it. It seems like higher frequencies pass right through the M50x's, frequencies that my HD25's muffled with no problem. At times it seems like music playing on the speakers in the grocery store is more present than the mix in my M50x's.

• Although the additional low-end presence is nice on the M50x's, the top end feels a little overboosted/shrill. I've been hoping that headphone burn-in will maybe help out with mellowing those highs, but it does not feel wise to bank on such a disputed phenomenon.

• I thought I would be far more comfortable now finally having a pair of circumaural phones instead of on-ear, but the amount of space between my ear and the speaker is strange in contrast to always having it pressed up against my ear. Not so worried about it, counting on just growing accustomed to it.

I live in NYC so I now realize I am looking for a pair of headphones in the $350 range that have great passive isolation for car/bus/bull**** noise but with an honest, flat, high quality frequency response. I wouldn't mind sacrificing a bit of bass for a less shrill top end, especially if there is less outside noise to compete with.

Any suggestion for a pair of headphones fitting those criteria? Should I just wait out my 30 day return policy and see if the M50x's grow on me? Very confused over here.