What about the guys who would daisy chain two or three of these together? You know back in 1967 there weren't many companies making great PA systems (they were just coming out), and even Marshall made a version of this same amp made to take hi-impedance mics (had eight pre-amps)
Anyway - the point is that back then all of your volume (from a guitar) had to come from your amp - not so much the PA system, so they really cranked them up.