Setting Sub Amp Gain With DMM

I'm new here. I've been reading all the forums everywhere on the net I could find. I specifically was looking for how to set my amp gain (monoblock subwoofer amp) using a DMM. I read a post here on this forum also. I've been asking this question in different forums but haven't found the answer yet. What I found is conflicting data. Some threads say to unhook subs from amp, turn HU to 75% with bass treble loudness set to zero, then hook DMM to sub terminals on amp and set amp gain to a specific voltage. (In my case 500w rms amp set to 90% = 450 X 2 ohms = 900. Square root of 900 = 30. I would set my amp to 30v output AC). I only want to drive my amp to 90% rated power. 450 watts with two 10" subs in a Jeep will be more than plenty for me. Other threads say to leave the speakers hooked up and use ear plugs when setting the amp to 30v AC. That makes more sense as the amp will be under a 2 ohm load. Wouldn't setting the amp to 30v with no load show a different gain setting than doing it with a 2 ohm load attached? Let's say I set it with the speakers connected and the dial on my amps gain setting was indicating straight up or 12 o'clock position. If I then set the amp with no speaker load present, will the dial on my amp still indicate 12 o'clock ? I don't see how it could... (no load vs load). I need clarification on no load vs load. Next thing. Some threads say to zero out bass treble loudness on the HU. Some say crank in the bass treble loudness as you would normally be listening to it THEN set the amp gain. Let's say I set everything on the HU to zero. I set the amp to 30v AC output. I unhook DMM, hook up my subs and now put in some music cd's. The sound is not to my liking so I crank up the bass, treble on the HU and flip on the loudness. Ahhhhh, now it sounds like I want it to. What did I just do to the 30v AC DMM reading? Didn't cranking in bass treble loudness on the HU effect it? If it didn't effect it, then why zero things out to begin with? If it did effect the DMM reading, then why not set the DMM with full bass treble and loudness ON ? I'm really getting confused the more I read. I truly want to set my systems correctly and enjoy years of trouble free music. I have 3 cars all with added sub systems. I want to be consistent in how I set all of them up. Thanks. newby Danford1