One way to "get back" a bit of quality can be to use an exciter (I say get back in quotes because it isn't really possible). I find warping in Ableton can result in the audio loosing some high end and loosing some high end harmonics, and an exciter can help bring back a little bit of that high end. I really like the Modern Exciter, included in the
Antress Modern Plugins free VST pack. lapieuvre's advice is spot on, otherwise rerecord!
One trick to change the key with little quality loss is to use the repitch warp setting and then change your track's BPM. Say the original track is in the key of A and recorded at 100BPM. You can get it to the key of B by selecting repitch in your warp settings and then changing your track's tempo to 112.25BPM (493.88/440*100). Note that this locks you into the new tempo however.
New Tempo = ((Frequency of desired root note)/(Root Note of Original track)*(Original track's tempo in BPM).
Use this as a guide for finding out note frequencies. In the example above I used middle A (440Hz) and middle B(493.88Hz).
This is like the effect of playing a 33 at 45 (if you know about records...), but doing it precisely to transpose between keys.
