"The sort of damage you describe can happen when a speaker lead is mixed up with an instrument lead and plugged into an input - of the same amp or another item. "

Well I did tell the boss "That amp did not blow itself". If thre guy did that it will be time for some customer education or something.

"If the sleeve of the jack plug is "hot", input ground tracks get blown open. "

Understood. Work on those big "BOOM mufuka BOOM mufuka" car amps ? Let the ground lead come a bit loose on that amp and BOOM is kinda the word. When that happens they get a letter printed explaining that if this happens again it will not be covered under warranty. Might have to do this in this case.

"If the tip is hot, the voltage can find its way onto the DC rails of the preamp and do lotsa damage. "

Well it could be hot with the output of a six hundred watt amp so yeah. But, it loks like the little fusible that supplies the preamp took a bad hit. should I assume the preamp is shorted ? Probably. Hopefully the thing has protection diodes and I don't have to change all the chips, hopefully just the regu, wait, does it even have regulators there ? Hmm.

"That IcePower module is tiny ( 4.3 x 3.1 inches) and lives inside a combo amp - right? "

Yup. The smallest 180 watt amp I ever saw. Even other class Ds in that power range. It's been a while so I don't remember the makes or models. Of course that means the switchers probably weren't good enough to just use the copper on the board as a heat sink. I seem to remember some of them using TO-220s standing up with only their tabs for heatsink.

That link to eBay is probably the one I found. I would really like to get one cheaper because that approches what this thing sells for. These musicians are not wealthy I have found.