I've been playing with REW on my laptop analyzing my subs' response curves.
Both of my subs have PEQs on them, and I was able to tame a couple of peaks that displayed themselves quite readily when charting both subs concurrently, but not when just testing one individually. Still, my room response chart wouldn't impress anyone with anything other than how loud and low they can go. Flat you would not call it. There is a big dip at the sub-main crossover point that is driving me nuts.
Anyway, I had the funky idea to test out the curves with the different native tuning selections - I had previously been running both at 16Hz. While on this testing path, I happened to chart the response with one sub at 16Hz and one at 20Hz - the results were remarkably flat and very impressive!
In short, the best thing I could do to improve my frequency response was to set one sub to 16Hz and the other to 20Hz.
Anything similar with any of you? Any ideas why? Comments?