I just posted this in another thread here:
the best thing to happen to my tone since getting a tube amp was gettnig an overdrive pedal.
It really focuses the tone into the mids and tightens it up, which makes it sound alot more like the tones Andy gets (as opose to sounding lose and imperfect without it).
As well as focusing the tone, what it does to the attack of the guitar is just incredible.