Well, good news, Rich. I've managed to solve the problem with no sustain on the MIDI-bass. Turns out I hadn't quite understood how it worked. Apparently, the lower register has no sustain at all, while the higher register has sustain but feeds back like crazy. The solution? I just used the middle register. It's still an octave lower than a regular guitar, and after adjusting a few parameters, the bass has a nice, somewhat warm and organic sound to it. It's not really very loud, though, but it works.

After getting the Bass right, I threw in the 4-bar drum loop I had, gave it a listen, and felt like it was missing something crucial to get that rock ballad-ish feel I was looking for, so I threw some acoustic piano into the mix (well, MIDI Acoustic Piano, at least), and gave it another listen. Sounds much better now. Almost like something I'd use for an intro to a rock ballad, which gave me a couple of ideas. First off, I'll make sure to save this track. Secondly, I'll try looping it a couple of times and improvising to it. Third, I'll listen back to it, note down which parts I find sound good over it and which don't, and then finally make the recording available for your listening pleasure... And hope you'll give it a listen, of course :) I expect the recording to be up some time tomorrow...