The question of current only occurred to me afterwards. I hadn't seen any measurements of zapper current mentioned anywhere, and concentrated only on voltage. Searching specifically for current measurements, I see that most zappers output in the micro- to milliampere range. I measured the stereo amplifier's current at 0.22 amps, hundreds of milliamperes, definitely too much current in retrospect.
Good points as well about the narrow bandwidths of cheap amps and their tendency to cancel offsets.
Trying this approach for a couple of days didn't kill me, but I wouldn't say it helped me too much, either. Lesson learned. At least the spirit of experimentation is alive in the modern age. If I break out in green spots, I'll be sure to let everyone know. :-)