unless theyre in the power stage, there aint much point in tubes.

i have two pro-co rats. theyre very, very nice. at low gain and with the filter quite high its a nice crunchy overdrive. at high gain with the filter down you can get crazy noise. i just bought a danelectro fabtone off ebay too, to get even crazier saturated noise.

overdrive is meant to be more like the sound of all tube amps at loud volumes, that natural singiness and crunchiness they get. distortion is a more abrasive, almost clipping sort of sound.

basically, overdrive is crunchy, fuzz is fuzzy and distortion is somewhere in between: a good distortion can crunch and chugg on low strings but sing on higher ones, always keeping definition. thats how i think is best to think of it.

but yeah. i think a rat is a very, very good choice, and theyre cheap on ebay too (dont worry about getting them second hand, theyre pretty much impossible to destroy). i got mine for about 40 quid for the pair. admittedly, that was a lucky deal, but you can probably get one for 30/40 quid if you look hard enough and are patient.

but yeah, as chris said, if you have a tube amp, you probably wont be able to beat cranking that for overdrive. if its too loud, get an attenuator.

probably the best non boutique fuzz pedal out there.