Frank,
As popular as your product may be, I am sure you get a few returns every now and then.
Can you tell us what do you do with them? If the people who returned them are careless and put a few minor dings or scratches on the exterior of the amps, or preamps, or worse yet, they opened the cover and messed with the parts and boards of the amp, and I am sure you salvage the parts, fixing them up, and resell the returned product, how do you prevent the next customer from getting an amp with a few minor dings and scratches.
I am very meticulous of what I am buying, every time I take a piece of audio gear out of the box, I look and see if the piece is perfect, I mean perfect. I can tell normal tool marks that were put on during production from dings and scratches put on by idiots.