You have to be careful about buying AV gear in the states. Yes, we're all aware that their power voltage and frequency are different (and many now auto-switch), but it doesn't end there. Their AM frequencies are spaced differently to ours. Their FM transmission spacings are different. They use NTSC, not PAL. Their RDS is different to ours, and they don't have teletext. Their TV transmission frequencies are different too.
Make sure you're clued up about all the American/UK differences before you go, because American salesmen won't have a clue about European standards.
P.S. The American equivalent of Comet is called Circuit City - they have a website which you can study before you go, but bear in mind that many prices will be internet prices, not store prices.