I have been playing a lot of triadic sweeps. I know that I can play Cmaj7 over an Amin7. I also know that I can play an Emin7 over an Amin7. I would think of that as Imaj7 over a vi-7 and a iii-7 over a vi-7. Any one want to hip me to more of those substitutions for you typical chords. A web source would also be cool.