audio enhancement [a little OT]

Well I know you can use wave shaping to increase the harmonic content of a low nyquist frequency signal (after its been up-sampled), and i _guess_ you could use some kind of expander to increase dynamic range (after increasing bit depth)-- are there any other, perhaps more sophisticated, examples of increasing the apparent quality of a signal?

I'm not after a max patch, but thought I could draw from the collective wisdom/experience of maxites in general. Any other techniques or suggestions of where to look out there?

thanks

tm