I really dread to think what mp3->ogg would sound like, even on my cheap amp.
I tend to lose a bit of bass whenever I do so, which ticks me off (my subwoofer always cranked, I like *feeling* the beats, literally, just hate when it shakes something small on my desk, makes an annoying buzzing noise), so I have to resort to keeping them either in MP3 format, or actually put my CD-drive back to use and rip them again, but in straight OGG.