Technically speaking, what is happening when you turn the subwoofer output on your deck up? For example, from +0 to +5. I know the gain settings of amps are important, does the subwoofer output cause any dangers of clipping, etc? Also, on Alpine HUs they have the 'Mx' option that seems to bring the sound more to life but also more sound from the woofer. Is there any danger in running the Mx setting? Just curious.
Unless you REALLY know what you're doing (not most people) you need to set the gain with the sub level all the way up. Doing it with that at 0 and then turning it up create the potential for clipping...
All that sub level does it incrase/decrease the RCA line level, the very thing you are trying to match with the gain on the amp.