I thought the Mass ratio thing was inherent to all solvers, about precision error accumulation, and all, and float/double imprecision weren't helping there, as simulation equation imprecisions cannot counter its bad effects. But It's 2-3 yrs without doing physics coding some it's from memory... You could try to revive that old thread http://www.bulletphysics.org/Bullet/phpBB3/viewtopic.php?f=4&t=650&hilit=Mass+Ratio and check if anything new solved that.
Love the midi physical knob idea, steampunk retro style tweaking, would make it really nice and fun.
Note that you could also hookup a http server https://code.google.com/p/mongoose/ and drive that from a web broswser (some code around about debug visu for that here http://www.altdevblogaday.com/2011/05/03/my-browser-draws-better-than-i-do/)
And there, in the web interface, you could put knobs https://github.com/g200kg/webaudio-controls ;)