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