You've got to remember that up to a certain point in amplifier history, engineers were trying to make it cleaner, louder and more effecient. Distortion was not their aim back then. Tweed and blackface circuits differ tremendously. I doubt if it would possible to sum up all the differences here. Some of them are that the tweeds had a different tone circuit which was usuall one knob. Blackfaces had 2, usually 3 and was considerably cleaner because you're actually losing signal in a blackface tone circuit. The single knob tone has more midrange 'cause all you're doing is rolling off highs or lows. More feedback was used in BF circuits too, which returns a portion of the speaker output to the (usually) phase inverter which helped smooth out an amp. Amps with little or no feedback sound "wide open". BF's sound, well, smoother. Many other changes were made as well. What I've pointed out is very generalized. There were variations in the tone circuits along the way too. The best thing to do is find somebody who's got these amps and listen. Maybe you know someone who works on tube amps who would let you hang out and hear these things for yourself. The technical aspects could take up pages.