This sub hums a lot when first turned on, and once in a while starts popping
very loudly shortly thereafter, at which point I have to turn it off. It seems
"unstable" at turn on, making funny noises before the popping. The rest of the
time, the hum fades down to barely noticable, and everything is OK other than
the low hum.

I talked to the manufacturer, who said this old design had this problem, and the
only solution was to send the amp to them for repair/replacement for $285. They
describe this amp as a "beer-can" amp or something like that, IIRC. The power
amp part is in an aluminum can of some sort.

If anyone has dealt with this unit and this problem, I would love to receive
hints for repair.

My first thought is just to replace the large filter caps and see if this helps.