HK 3490 receiver with no sound?

It's the classic case: I sold my 3490 that I had been using and it had been working fine. The buyer set it up and all functions worked but there was no sound. It has pre out and main in, and I don't use the factory jumpers but instead a better quality IC. So when I packed it, I inserted the jumpers. Buyer sent back the amp. I set it up, all functions worked, but no sound. I checked factory jumpers, they were inserted incorrectly; I corrected the placement; and the amp worked fine. After the fact, the buyer said he tried every possible way to insert the jumpers. He lives in a 100yr old house and plugged the receiver into a power strip along with a subwoofer and TV.
Any reasons other than the jumper screw up for the malfunction? He wants to try the amp again.