I think the reason so many people want old amps is because they do a sound very well that bass players back in the day hated, but modern ones love, I'm talking more about heads than cabinets here. Take Jesse Keeler from DFA 1979, he uses an old Peavey Festival head that I've seen people who played it back when came out have nothing but hate for, because it distorted and sounded bright, which wasn't what your average 70's root note bassist wanted, but it suits him down to the ground because he's a very different player. He has an original cabinet as well which he's replaced the speakers in with PA ones, leading me to believe that it was speakers that caused most of the problems for bassists back then rather than amps.