Oh dear! That is not good news. You just don't use amps that don't have speaker protection. To do so is inviting a blown speaker every time a power transistor fails. Line fuses just do not protect speakers from this event. I have doubted all along that your conditions of use damaged your speaker. My experience and intuition tell me that amplifier is likely guilty of the crime.
I would not use any solid state amp that did not have good speaker protection, that did not have either an output capacitor or an output transformer. At the power of your amp I very much doubt it has an output capacitor. If it, did the tech would not have tested it for DC off set Only Macs I think have transformers. Producing that amp like that was fraud, period.
I will file that amp for future reference and put it on the avoid do not use list.
Sorry to be blunt, but that is the best advice I can give. That amp belongs in the recycling center, it is a rogue design.