Typically for a grungy sound you want a very clean 60's voiced amplifier like a Fender Twin, or if you prefer a gritter clean (Kim Thayil, early Nirvana) you will want more of a Fender Tweed type sound that's slightly overdriven.
The distortion is typically provided separately by some kind of higher gain Fuzz or cheap Distortion Box (DS-1, Big Muff, Univox SuperFuzz - or something similar). Usually with the mids twisted up if you are going more for a out-front sound (ie Nirvana's In Utero or Soundgarden Badmotorfinger), or twisted back if you want a muddier/dirtier sound (think more Sonic Youth or Smashing Pumpkins pre-Infinite Sadness).
