Alternator Replacement

Unlikely the amplifier caused your alternator to fail. If your battery was staying charged over a week of driving, your alternator was doing its job. Not sure what the metal powder on the back of the alternator is. Perhaps it is just oxidized aluminum (white) from the case. Doesn't sound like a typical failure mode. Why do you think the alterator is not working? The dash ammeter (charge to discharge marks) is for monitoring that.