I'm new on the forum but I can jump in on this. Some amps label the gain this way, it's referring to the expected preamp voltage. If RCA voltage is 5V, you'll obviously need less gain than a 2V signal. It's probably a better way to label the gain, in my opinion, because it more accurately describes what it's actual purpose is.
yeah it's fine just set like normal, use 50hz tone with meter set volts to watts u want (make sure amp does watts u set it for fyi mb quart does not even come close to doing ratings., look at the crunch 1500.1 that tested 323rms 2 days ago.. made by same company)