Car Stereo question - system callibration?

I'm trying to set up my system which includes an active crossover - the manual for the crossover says to "turn up the output until the signal distorts, then back it off a bit"; the manuals for the amps both say basically the same thing: "turn up the gain until it distorts, then back it off a bit" ; fair enough, BUT . . . how do I know whether the distortion is from too much output or too much gain? (If I don't know which is which, I can't correct the problem!) Is there some rule of thumb I should follow regarding where to set the gain on the signal going into the amps, or should I work at it from the crossover end? I've gotten nowhere with Tech Support from the manufacturers of any of the equipment; their techs don't seem to be very knowledgeable, and think that reading the manual to me over the phone will solve the problem. I've read the manuals, they don't contain the information that I need, that's why I called the Tech Support line. Anyone have any tips, tricks or insights?