My "I will not modify the ti any more" mentality has long since been ditched at this point. I wasn't too happy with the stock sound system. It had an aftermarket stereo, but no bluetooth or USB, so it had to go.
I scored a cheap subwoofer and amp off a friend of mine. It actually is the same system that I pulled out of my E30 two years ago. He bought it, stored it in a closet for two years, and I bought it back. It doesn't look great, but it was cheap.
Still tracking down this misfire though. That has to be resolved before I do anything else.