Rick, I'm not familiar with what was on Youtube, and I'm curious if the volume control setting on the sub was thought to be crucial. Generally, I'd think that an initial setting near the middle would be appropriate, and if the calibration resulted in setting the sub trim at or near the minimum(or maximum)then the sub volume control should be set lower(or higher)so that the final calibration result is closer to 0.
Incidentally, it's a volume or level control, not a gain control. Home audio amplifiers have a fixed gain, and what that control does is to vary the amount of voltage that's let in to be subjected to that gain.
