I see your point to a point and it makes sense.
Here's where I see the challenge and I've run into this before with some of my previous systems from way back. You measure the sub only... then the sub + mains and see a 6db peak at 60Hz that appears with the mains, but wasn't there with the sub only. So you try to filter the peak at 60Hz but nothing moves... it just stays there. Of course you can try phase adjustment and it might help.
I found myself aggravated to death one day... I had a serious peak at 100Hz that I thought the sub was causing. I ultimately filtered it to -48db and it didn't budge. I was :scratching: ... then I figured it out... ahhh, my mains were still on and I wasn't paying any attention at all to that fact. I was trying to adjust something that couldn't be adjusted. If I remember correctly I was able to help that peak with the tone control on my pre-pro at the time.
But generally I've helped a lot of people adjust the sub only to flat or a house curve and the majority of the time when the sub + mains is played, it looks good. Honestly, I can't even remember the last time it was an issue. Again, I'm not gonna say that it's not an issue and I am definitely not gonna suggest to anyone that it's not necessary... they can proceed as they wish. It certainly will not hurt to measure both ways prior to eq'ing...
