Right now fb has nothing to do with it. It is not your fault that he took so long to get back to you- not your fault if the amp sat damaged in a box until he got around to it or back in town or whatever. Its irrelevant.
If the claim time has elapsed that is all that matters at this point. If you packed it incorrectly it does not matter, unless you know that and want to make it right. If he unpacked it correctly it does not matter. What matters is that 10 days elapsed before he contacted you!
We do a lot of shipping and occasionally we get a damage claim that we have to sort out. I can't think of a time that has happened wherein the customer took more than a day to let us know there was a problem! IOW the time frame is fishy.