I would never shop any place that said that.
it really is the equivalent of saying:
BENCHPRESS 300 POUNDS!
and expecting it to somehow be easy simply because YOU WROTE IT IN BIG LETTERS.
basically, the rca signal just tells the amp what to do. the gain control is there to allow basic volume adjustments, and to allow different equipment, with different output voltages, to work well. but the things that make the amp hot really aren't related to the input signal (the directions), but rather the output (performing the actual task).