Any idea what might be causing the awesome gritty distortion in this? http://www.youtube.com/watch?v=2D1oVRdV77k&feature=related

It's some people paying a tribute to RL burnside and doing a good job covering his most famous song so i wont be able to find out what was going on with the sound in the video. It almost sounds like a small mic being clipped like a computer mic or maybe a razored speaker. Any ideas?
Sounds like it could be a little speaker like one of those in an old Fender Champ being pushed to the brink. It's sexy!
I agree, sounds like a little speaker being driven hard in a small cab - reminds me of my transistor radio I converted to a practice amp..