This is awesome. There was a block letter 5150 at the place I used to practice with my band, but never really liked it's lead channel. It was too damn gainy, and I ran the pre on like 5, so I would use the available Orange TH30 instead, or even the solid state Crate head.
Then one day, I had no choice but to use the 5150. This time however, I tried the rhythm channel, but it wasn't mean enough, so I pushed the crunch switch, and it was perfect. Just perfect. It's probably the best tone I've ever heard for heavy riffage.