Yes, bitrate.
Euh, wav is cd format, that's lossless. mp3 is with loss, so much less in quality then wav or other types of lossless format (like FLAC, ALAC f.e.).
The bitrate of an mp3 file is pretty crucial to it's sound quality. If the bitrate is 128, it's very noticably less in quality then the actual recording. Not worthy of headphones like your AD700.
192 is a little bit better, but if you want to enjoy sound on cans like AD700 or higher, I'd say you need 256/320 bitrate or simply lossless.
Get into this if you want to enjoy music with good headphones, otherwise it's pretty useless, to be honest with you.