I find the idea of using an EQ like this interesting. I currently use the the Behringer BFD to EQ my sub, and it made a noticeable difference in the overall response of my subwoofer, much smoother and flatter once I got the BFD dialed in.
I've considered the possibility of EQ'ing the mains, but the reason I've held back is because it would mean adding an extra Analog/Digital/Analog conversion to the output chain, which I can't help but think would have a negative impact on the sound quality. After all I recently spent what was for me a small fortune moving to separates, including a pre/pro whose sound quality I'm quite happy with. I wouldn't want to degrade that by adding a digital EQ with questionable DAC's and/or ADC's to the mix.
