BASIC STAMP is an interpreted hardware device which is very slow. BASIC and C compile to assembly. It is rarely the most efficient code in terms of space or speed for any given task, but it is easy and portable. Assembly instructions are different for different parts, so you have to learn new bits of language for different parts or "forget" ones which aren't implemented.
All emotional attachments to assembly aside, a prime question is, do you NEED to get the extra speed/compactness of handwritten assembly? A PIC18 can give you 10,000,000 instructions/sec, and a 32k code space. I'd have a very hard time believing a MIDI application could not work within this space just because of the code overhead from the C compiler.
You can get "trial" versions of C compilers from HiTech.