http://www.mesaboogie.com/Product_Info/Rackmount-Preamps/V-Twin/v-twin.htmlMesa v-twin.
Theyre out of production, so you can get one used for a decent price.
Now, this thing can go from extreme cleaner than clean, all the way to brutal distortion, and everything in between.
My old guitar teacher had one, and used it to make his Tele and gibs melody maker (with fralins) sound cleaner through his fender princeton. Now, this sounds noobish, but hes been playing for almost 40 years, and i trust his judgement. He uses it for pretty much all of his amps... heres a list.
Fender princeton
fender bandmaster (beige with oxblood?)
http://globalvintage.com.au/shop/images/DSCN2129.jpgfender champ 50's (the orange tweed one)
fender champ 70's (the silverface one)
and that pedal cleans all of them up SOOO well. Its incredible. It can also turn any of them into death metal machines :D