Alright, here's my second go at this. Since moving this big ass sub from where it is now isn't an option, I tried a couple of other things, First I plugged two ports on my SVS and tuned it down to 16hz, not really to help the null just because it makes me happy :p . Second I added a second smaller sub a 12'' Infinity, I can't remember the model number, too see if that would help any. Then I tried bringing down the other frequencies to match the null and raised the volume on the SMS-1 to match my receiver, to get the head room many of you speak of and to keep from stacking the frequencies and here is what I came up with.



Sorry for the blurry image.

Looks better than mine.
Does that volume setting say 40?
If I had to make any comment at all (and this is nit-picky) is that your subs appear to be just a hair louder than your mains. I actually like mine that way so, if it were me, I wouldn't change anything.
My curve looks more like your first picture. Hmmm, I feel a bout of SMS-1 tweaking coming up...