Hey Jason,
was appalled by the sound of the mp3's through a good system.
MP3's are as good as what the original copy was minus the amount of compression. For a reasonably good copy it should be at about 128k. 300 k and up is becoming overkill. 128k refers to the sampling rate.
What happens is; to shrink info, they truncate the music information above 16khz. That reduces the file size then its compressed. We can't hear above 16khz anyway. Microphones can pick it up if they are a super quality. Speakers of high quality can reproduce it. Really makes almost no difference to the ear.
MP3 at 128k rate has no info above 16khz. It is about the lowest to go for Hi Fi. MP3 at 160k does have some info above 16khz. 220k has more.
MP3 rate below 128k is no longer Hi Fi. and is done because the file is smaller. and more files can be placed onto recording mediums. At 90k music quality suffers and can easily be detected on average Hi Fi systems. Below 50k voice starts changing distinctly.
There are programs that you can use to check what the compression ratio and quality happens to be.
Original CD recorded quality can vary as well. That is a bit different.
So MP3's can maintain all of the original recorded quality. But that depends on what rate it is compressed to.
If you like PM me and I can help you out a little. I have a number of relevant tools in my armoury.
I can even check the recorded quality of tracks on CD's.
Cheers Marty