I got a great deal on a used Boss RC-30 looping pedal recently, and I've been noodling around with it and my Etherwave. I'll post some SoundClouds once I've had a chance to get to know the pedal better.
I'm curious to hear what songs you've tried with loopers & theremin?
So far, I'm finding it run to lay down a bass line by overdubbing a single note at a time. The great thing about the RC-30 is that while the phrase is looping, you can find the next note to record, get your timing right, and then tap the pedal to start/stop recording around the note. This works great for setting up pre-recorded phrases, but I haven't worked out the timing & technique yet to be able to do it "live" as a single performance in a musical-sounding way ;)
It also has some built-in effects and rhythm tracks, but they're pretty basic. You can connect it to a computer via usb and upload your own .wav files though, so the possibilities for using it as accompaniment during live performances are virtually endless.