I for one would be really interested in hearing the results of actual analysis of the preamp and amp. There are lots of great theories here, it would be really be good to hear what actually happened.
I don’t think the take away is to go buy a VARIAC. First, I have never heard of a preamp frying a set of speakers. But I am not versed in practical electrical engineering. But, even if this is a thing, then the take away is to buy from a reseller that verifies it functions properly (like Echo… and a host of others) or to have the device checked over by a audio shop.