Hi Fraser, I agree that Amadeus II is good but the thing I've been using quite regularly is Audio Hijack Pro which essentially records whatever you playback on your Mac (either computer-wide or app-specific). Once you hijack your output you can apply a large number of Soundtrack Pro or Logic-like filters which can really juice the output quality of an MP3, allowing for a final tweak in iTunes or Amadeus (or Compressor if you have it). Oh yeah and you've gone ahead and bought the SRS iWOW for iTunes to improve output definition and focus -- rrright??? (well worth it...)

Cheers