If you want to get a ballpark on setting the gain correctly, a voltmeter can get you close, assuming your amplifier is rated correctly or underrated. Simple power equations, how much power is your amp rated for, at what voltage? After that, you'll need a dummy resistor.
It doesn't say how much power at what voltage on the websites specs... just the power at the given impedance.
Per channel Power:
125 W X 2 @ 4 Ohms RMS
250 W X 2 @ 2 Ohms RMS
500 W X 1 @ 4 Ohms Bridged RMS
1500 Watts Peak/Max
Given this information, and given I have access to a DMM, is there any way I can either set the gains properly or be able to tell if they are set improperly?