@ Pete, that's what I've been working from. I was not able to get rid of that null by adding subs. Not to say it would be impossible to do with subs, but I'm not interested in adding any more subs at this point. I get that null with just the 12's and just the 18's, so in theory combining them is not going to solve for that problem.
I'm finding a lot of ground up threads. I'm looking at this as a $500 problem to make a first impact and a $1000 problem over 6 months to fix. I'm not interested in refinishing my basement. The graph shown is 2 12's running in phase with one another grouped together under my television directly across from the MLP. I pulled it out and swapped polarity of one of the subs to verify phase was correct. As best I can tell, it's not a phase problem.
I'm not against DIY if it accomplishes the same thing for less money. It seems like the roxul Rockboard 80 is the best bang for the buck. The Gilford of Maine fr701 seems like the go to in covering. I've read a few DIY tutorials, and it looks within my ability to pull off.
That being said, the GIK stuff seems pretty reputable and depending on how much I need, might not be too expensive. The acoustic room kit number 1 would be in my budget, and then I could DIY some panels down the road if necessary.
looks like the tuning might be a little off, but the dip doesn't appear to be a problem with the box or a combination of the two subs fighting, but a problem with the room.
Couple of questions...
I could have a 4" gap between the subs behind the couch and the wall it sits on. Is there any benefit to filling that gap with rockboard? If the subs were pushed up against the wall, would that be better?
Most of the threads I've read have been focused on an overall sound improvement. I'm really happy with how well Dirac has tamed LRC and surrounds. I'm not necessarily looking to fix those things. Is there a way to focus on this gap between 15 and 20? Am I reading the graph wrong? My assumption is that room gain is pushing the graph into the 15,16,17 range and that the null is chopping into that. The 18's get into the 12,13,14 territory, so I have the potential for some great low frequency stuff, if I can fix that notch. When I mix the 18's with the 12's the dip at 90 goes away, and if I cross at 120hz the 125 dip isn't such a big deal. Obviously if all of that gets cleaned up in the process, it's a win, but I really don't want to spend a grand trying to fix the low stuff, and not be able to do it. Any advice here would be greatly appreciated.