The beauty of these boxes is alongside the crusty, dirty sound, once you start maxing out the feedback level they start to scream. Normally this just makes a racket (not a bad thing around here by any means) but when a percussive loop is sent through, the howling feedback starts to latch on and pulse along with the beat (normally the hi-hat patern). Of course this doesn't happen every time and you have to keep an eye (and ear) on the levels but with a bit of luck you'll get an interesting effect.
Perhaps our favourite box for this kind of abuse is the good old Realistic/Radio Shack reverb. Really more of a tight slapback delay effect than a true reverb , we own 3 of these beauty's, two we circuit bent into howling monsters, but the one above we've left untouched. Dig those faders!
To give you an idea of what we mean here's a short video of our Korg Rhythm 55 and the Realistic doing their thang.