What sort of distortion? Super-heavy and chunky or crunchy and bluesy?
The best medium gain tone i've ever heard (and, come to think of it, the best clean tone i ever heard) came out of a Dr. Z head with a Fulltone overdrive hooked up in line. Pushed, the amp gets this great bluesy overdrive- neil Zaza once descrived his as it sounding like "angels flying out of the speakers," and i could do no better myself, and throw the pedal in there and you get this sweet lead tone.
Actually, come to think of it, I'm in the market for a new head too. Ought to call the guy up and ask where he got his, i was thinking more of a "high gain" amp, but that and a great overdrive and i might be better off... the thing sounded glorious.