There are a number of things that contribute to that "good distortion tone", as Atomic put it. And it's no coincidence that a blackface Fender Bassman was the amp that provided it. The blackface Fender amps (Bassman, Deluxe Reverb, Vibrolux Reverb, Pro Reverb, Super Reverb, Showman, Twin Reverb, among others) are the most sought after vintage amps for a reason - TONE ! In a tube amp, the types of tubes used and their condition (both power and pre-amp), the bias setting, the internal component wiring, and the types and sizes of speakers all affect the final output. Depending on the types of tubes used, the bias setting ("hot or cold") and the speakers (2x10" 4x10" 1x12" 2x12" 4x12" 1x15" 2x15" with either ceramic or alnico magnets), the Fender blackfaces can range from extreme clean at loud volume (Twin) to early "breakup" (aka distortion) at lower volume (Deluxe). (You'd think I work for a Fender vintage shop !!!). Fender got it right back in the late 50's through the 60's. BTW, their Reissue series of these old amps illustrates the high regard the old blackfaces are held in (they come close to the originals but ... they're not the orignals).