Just out of interest what kind of file limits are you working to for each song? I had a quick go at trying to replicate the original MegaDrive sound by sampling the original FM instruments directly and managed to squeeze something okay-ish sounding to within 64kb for Rise or Fall (which is a comparable size to Amiga 1mb 500 games of the era such as Jaguar XJ220 and Lotus III). I assume you're having to work with less than that?