MP3 is weird. I find the change in spectrum can be very noticable. It sounds as if the mp3 has encoded a small number of peak filters and reconstructs the sound from those, especially in subtler multitimbral harmony. Sometimes, undertones which I'm used to hearing as a part of the music are missing entirely. I think my ear adapts more easily to an EQ or change in reproduction because the spectral changes are usually broader and smoother.
I've only ever heard WMA once, and it was even weirder, it seemed to drop mid range in favour of a punchier bass. The rhythm guitar almost dropped out of the mix.
I like and recommend the Ultrafex Pro. I have trouble hearing voices above other sounds. It solves the problem. (I first used it at DWCon.)
Distortion from a high-z joint is one of the most annoying things ever. White noise is comforting.