Like most things in life, you play your own hand. At this point, certain facts have occurred and this really boils down to your desired outcome, which you can largely influence.
On one hand, you have your money, the buyer has the amps and a "copy" of the manual, and you can simply say "enough," or "piss off," - your choice. As Lloydc indicates, you substantially performed and damages are likely limited to the potential loss of resale value due to the missing "original" (vs. copied) manual - e.g. small. The likely outcome with this solution is that you will receive negative feedback and the buyer will not bother taking you to court over a stinkin' owner's manual.
On the other hand, you could ask the buyer "how can we resolve this so that you are satisfied and we simply forget feedback?" Be ready for requests ranging from a partial refund to return of the amps and full refund. This outcome probably results in some level of hassle and pain to your wallet, but does not affect your feedback, assuming the buyer agrees.
The cards are dealt, it really boils down to how you want to play it.