bigshot
Headphoneus Supremus
I don't mean to offend you or doubt about your ears, but some of us can hear the difference. It became apparent to me by comparing some tracks and some whole CDs I previously ripped in 320kbps MP3 after I re-ripped them in ALAC.
Plain vanilla MP3 is a very old codec. There are newer ones that are much better.
Try 320 LAME and balance the line level. Compressed files are often a little quieter than the originals, presumably to avoid clipping during compression. Also try AAC 256 VBR. AAC is as good as LAME 320 at 256. Both of these are better than 320 MP3.