It really depends on how the mp3 was created. The higher the bit rate, the less information is missing from the track, the higher the perceived quality.
I remember reading a good idea - probably here - that involved making a CD with a single song, but with multiple mp3 bit rates, then playing the CD randomly in a "blind test" fashion, to see if you can tell what bit rate you're listening to without looking at the track number.
I only use mp3 for my (now old) portable player which I only use on planes - a decidedly less than perfect environment. Everything else is store is flac (fully lossless compression). Drive space is so cheap.
Why would you ever want to listen to lower quality recordings with such good speakers?
Try out a 128 bit mp3 for a song with a lot of high frequencies, cymbals and the like, then try a 256 or 320 bit file. I personally find that the dramatic increases in sound quality stop at about 192 bit, and it's pretty incremental after that.
Edited by Kruncher (01/16/09 06:43 PM)