I mostly use an Xotic BB Preamp, set for a reasonably crunchy sound, through a two channel amp - gives me a crunch sound on the clean channel, then a really overdriven sound on the dirty channel.
When I want a stupidly heavy distortion sound, I use a Seymour Duncan Twin Tube Mayhem. I did try a Blackstar 2 stage pedal, but for some reason it made my wah pedal squeal whenever I switched it on. :?