Thanks everyone for the support for the PR sub I brought with. The woofer I put in the box was the AV15X. The pair of 18" PR's were the 1600gram model which tunes the box right around 20hz. It gives up a little sub 20hz extension, but gives more efficiency in the 20-50hz region this way. This would be the better option for music only. Music has very little material below about 30hz. We saw proof of this as there was very little program material that really moved the PR's, meaning we didn't approach the 20hz tuning much. The AV15H with pair of 2500gram PR's would be tuned to around 16hz and give more output below 20hz for movies.
The cabinet was a 24" cube with full 3/4" MDF on all sides and had a single brace top to bottom. There isn't much else to brace as you have a woofer on front, PR's on sides, and often a plate amp on the back. Attaching the three cast aluminum frames to the cabinet also increases the stiffness. It was finished with a new paint we have made and will be selling once it warms up. It is water based, very low VOC's and is ECO friendly. It can be sprayed with a cheap pressure pot from Harbor freight or rolled on to also get a nice texture. It is extremely high build, self priming and sealing so you just put it directly on MDF or baltic birch with no primer. The only thing that makes it a little difficult is that being water based I can't ship it in the winter. The "feet" were actually the phase plugs from the TD10's or TD12's. I couldn't find any rubber feet the night before and remembered Nick had once used the phase plugs for feet. It worked out quite well. It's nice to be able to get your hands underneath to lift it without worry of crunching fingers.
Regarding the output of the sub, I'll admit it did have an unfair advantage over all the sealed enclosures because of the PR's. It was also greatly handicapped by the placement on the side wall with a doorway opening at one side. A sub with dual PR's on the sides or multiple drivers on different sides tends to benefit even more from corner placement. That is one of the very difficult things about auditioning the subs. The response changes significantly with placement. With so many subs though it would have taken forever to keep switching them out and putting each one in the corner. Patrick if you get a chance play around with placement with the same sub and you'll see huge differences. Also if you get a chance, try running the AV15X up to 500hz or higher. I think you'd be surprised at the quality of the lower vocal region that can come out of this "sub".
The following graph shows 4 curves just as a reference
Red is the AV15X with 1600gram PR's at 20hz tuning
Green is the AV15H with 2500gram PR's at 16hz tuning
Light Blue is the Mal-X 5cf sealed (i think that's about what size those were, correct me if they weren't)
Dark Blue is the LMS Ultra 5cf sealed
The AV15's with dual PR's in similar enclosure volume will of course be more efficient at the lowest frequencies due to the nature of the PR's. The bottom graph shows the maximum acoustic power output. This is determined both by displacement and power handling. Between about 17-27hz the AV15X with PR's has a significant advantage in output capability over the 18" woofers. This is because each 18" PR individually is capable of displacing more air then either the Mal-X or even LMS Ultra. There is a point though where the curves cross as the response drops sharply with the PR's and below that the larger sealed drivers excel. It's all about how much air can be displaced at a given frequency and determining which frequencies you determine are most important. Then factoring in how much enclosure volume and power are required to do so.