Motorboating is due to a low frequency feedback path in the amplifier circuit. It has little to do with power supply, though the feedback can be introduced through accidentally poor value RC or LC filters between stages (much as a phase shift oscillator works).
Just because an amp cost something, or has some weight in its name, means absolutely nothing. The highest end amps are just as often, just as trash as the lowest. This is not my experience directly, but a close friend does tube amp repair (among other things) regularly, and this is his experience.
He often fixes design faults in these things, as well as incidental repairs (burned parts, etc.). (Most of which are caused by design faults anyway, i.e., that could've been reduced or eliminated through proper design. Electrolytics placed beside stinking hot vacuum tubes, for example.)
(The low level 100Hz ripple also suggests poor design, either in filtering, balance or shielding.)
Tim