[Wayne A. Pflughaupt]

I guess that is a question I have then. Why don't you just make a flat response and then increase the level of the subwoofer over the mains using the receiver sub level control or the actual sub gain control when needed such as when watching movies but when listening to music, decrease the level so that it is flat, which I read that a lot of people do?

I can’t speak for everyone, but I found with music, when the sub was flat, that the bass notes got weaker the lower they went. That’s why I curved response upward, so that the relative volume of bass notes remained constant as they went lower. Everyone’s room is different, however. If you’re getting equal fullness with both high and low bass notes with flat response, then you’ve achieved the objective.

Isn't this what most people do anyway, i.e. increase the level of the sub 3-4 dB hotter than the mains for watching movies?

Typically so, often even more, depending on the size of the room.

Regards,
Wayne