Assuming you have the seller's pledge to refund your money upon return of the amp- through email communication (preferably ebay's messaging system) you should be fine. Return the amp to him as he requested- then it's his baby to sort out if and when he can. Whether he is successful in filing a claim against UPS will then become irrelevant as far as you're concerned.
You have little to fear as far as getting your money returned from the seller: Ebay is highly aggressive in safeguarding the purchaser's interests and will debit the seller's PayPal account and credit your's for the sale amount should he fail to uphold his end.
You should thoroughly document the damage to the amp (ie: photos) prior to return as well as any precautions you have taken to ensure no further damage occurs, and of course the shipping receipt(s).
From everything that's been said so far, it doesn't sound like you should have any difficulty. If the seller has solid transaction/feedback history, so much the better.
**When you return the amp, you should absolutely ship it a proper McIntosh carton available from Audio Classics NOT the stuff Mailboxes used. Advise the seller that it would be in best interest to have A/C send you one at his expense. That way his own case against UPS will be strengthened.
Here's my progress so far:
1) contacted the seller through ebay email and informed him of the damages.
2) Received a response email within an hour saying they were regretful of the problem and will resolve ASAP.
3) received a call within the next hour from the Mailbox etc office and stated that they are filling a claim with UPS for the damages and UPS will contact them, or me, can't remember, about a time for them to come by my house to inspect the wreckage and to take it with them.
4) Seller emailed me to call him.
5) Called the seller, he was very sympathetic about the situation and explained that he made extra effort to ship through this company to get it to me right without damage because of the weight and delicate nature of the instrument.
(even though he ignored the shipping instructions I gave for securing the amp on a plywood board and double box). He stated that I will be refunded through paypal once he receives the amp back from UPS.
He requested pictures of the shipping box and amp which I sent.
So what I think is next is just as he explained, wait for UPS to take away the amp and ship it back to either the seller or the packer, wait for seller to refund the money by PayPal.
I'm thinking of emailing the seller to verify our discussion and get it in writing that he will refund.
I don't want to file a claim with ebay until I need to but I run into this time limit thing quoted on their website:
"If 7 days have passed since you opened the case, and the seller still hasn't resolved the problem, we recommend that you escalate the case to eBay Customer Support. eBay will review the case and may provide a refund of the purchase price plus original shipping."
I know this will take longer than 7 days to resolve to my satisfaction!
I'll keep posting as things go on to for those who may have this experience in the future.