What to do when buyer claims sellers item broken?


I recently sold a Bottlehead Paraglows amp with AVVT 2A3 meshplates on Audiogon. The tubes were triple boxed and shipped separately via USPS Priority insured. The amps bases were shipped separately via Fedex Ground insured. I was very careful with all my packaging. I know these amps work perfectly before I shipped them out. Now the buyer received the amp and the tubes, and claims the tubes glow up then died. This is the first time I have had problem with selling my equipment, and I am not sure what to do here. If the amps and tubes arrived without any visible box damage, how can they be broken? There are minimal circuits in the amps to be broken, especially inside a wood box covered by thick foams and double thickness box. Please understand that I am not saying my buyer is doing anything to it, I just need advice on what to do next to rectify the situation. I don't sell a lemon, and I don't want a negative feedback from any buyer, ever.
PT
pt999
!st you have to realize things do happen in shipping sometimes that aren't seen. Happens all the time. Then you have to eliminate the buyer setting it up wrong,not knowing how to use it, etc. Then you have to consider things like the buyer trying to find a way to get out of the deal after having a free trial. Could be these things and a lot more. THe only thing you can really do is take it back, check it out and sell again.
The first thing I'd do is call each shipper and notify them of your upcoming claim. Presumably you paid the shippers, so their contracts to perform are with you. Each shipper will create a claim number for everyone's reference. Next ask the buyer to get an estimate for repairs. Then ask the buyer to submit the repair estimate to the shipper, referencing the claim number. If all goes well, the offending shipper will settle with you and refund your shipping fee. Then you can settle with the buyer.
PT,
I believe that it only happened on one of the amps
Did you ask if he/she accidentally connected wrong speaker?
This is the most horrible situation for the tube amp since in this case time delay of fuse might be greater than tolerance to keep such load for tubes.