It's not the frequency response per se, but I definitely hear a kind of residual noise or distortion (I call it "wurble") with mp3 and other highly compressed formats that is not there in the full resolution wav equivalents, and find it irritating. Maybe it is just me, or maybe there are better and worse mp3 algorithms, even at the same bit rate... of course bit rate matters as well, all mp3s are not the same. Many people say 320 kbps mp3 is acceptable, a lot more so than 128 kbps anyway.