I have never seen comments on the question if the gain of the sub amps needs to be the same as the mains and the relationship to the driver efficiency.
Does this mean the perfect amp is one with the same gain as the main?
Let's pretend the sub and main have the same efficiency. They won't, but let's start there.
Main amp is small, it has a gain of 23, Sub amp is big. It has a gain of 29. Both are typical gains for amps, though around 26 is "normal"
You adjust both inputs so that at 1 volt in, you get the same power measured.
Now put 2 volts in. Because volume controls adjust level, not gain, you get four times the voltage applied to the sub as you do the main.
Throw in driver efficiencies, and it gets ugly.
BTW, I found the cheap O-Audio plates to work quite well compared to others I have used. I have to admit, two of my systems have a Parasound
and a Hafler traditional power amps powering them.