Was your amp signed for? Generally, with a credit card chargeback you only need to provide them a tracking number to win your case. But that chargeback has to be filed within X amount of time (I believe 90 days?).
Forget eBay and PayPal as it's outside of case range (45 days). BBB will only tarnish their record. If you've given up all hope, turn to the company's state Attorney General. They will put the hammer down on any type of interstate commerce BS and shennaniganary.
(fyi, ripoffreport and resellerratings have customers tear down retailers, then extort the sh*t out of those companies. Basically charge retailers to keep their "record" clean, then skyrocket the prices or unleash every case they've withheld (very illegal))