I've been using a guitar amp with my theremin, and have been not entirely pleased with the "clamped" sound I get from it. However, it has some built-in effects that can get nice and nastily fuzzed out, which works well in some of pieces of the ensemble I play in. Anyway, based on a lot of research and some recommendations here at Theremin World I acquired a new PA/keyboard amp, a Behringer K450FX. I really like the sound from it. It also has preset effects, some of which are awesome (the "1 octave down added voice" is phenomenal in the low range!). However, what it lacks is some really nasty, dirty fuzz effects that would go well with some pieces I'm trying to work up.
So my question for the more avant garde out there is what have you found, either commercially or DYI, that can make a theremin sound really grungy/nasty/dirty? I'm moderately capable with a soldering iron so can make my own if it's not too complex. Any hints or pointers would be appreciated.