Kevin - Your Maelstrom project looks pretty good and I will be using two subs. In what ways would Mark's dual 15" sealed subs do better than your Maelstroms and what advantage might your Maelstroms have over his? The Definitive Technology Trinity subs have dual 14" drivers with four passive radiators so it seems they would have significant output as well. I've also looked at Acoupower drivers (www.acoupower.com), but it seems they do not have detailed cabinet construction designs for their drivers. I can follow directions to build, but I'm not a designer.
It is speculation without measurements but generally, you are going to get more headroom from the dual 15"s high in frequency because that is where the system is power limited and the two 15"s will have a cone area advantage (800cm^2 x2 = 1600 cm^2 vs. 1200 cm^2). The two separate coils and motor structures will be more capable of dissipating heat so you have more ultimate headroom where the system is power limited (I'd guess > 35-45Hz).
Down low, these systems become mechanically limited. When you pass into that threshold the Maelstrom system will have an advantage due to the dual 18" PRs providing much of the output. I don't know the exact size of the dual 15" system but box size is really what sets the system efficiency down low. If you put more drivers in the box, and scale cone area accordingly you have to scale the box size in proportion to keep the same bandwidth. That is one of the fundamental issues with big driver in small box type systems. You loose bandwidth and to get it back you have to equalize (put more power into the driver) to get it back.
One other note, as you pass below the tuning frequency of the PRs (about 16-17Hz) the Maelstrom-X system would start to unload. Somewhere in the really low subsonic content the dual 15"s will overtake the Maelstrom system once again. You will have to have a subsonic filter on the PR system so that curtails some of the really low frequency output (say 14Hz & down). I don't really see the point of 10Hz bandwidth but a lot of people focus on that goal.
Of course if sealed is what you like, dual Maelstroms is an option. You would get more output than the dual 15" system easily. We all play by the same set of rules.
By facing the drivers in opposing directions you cancel out much of the cabinet vibration.
Also, if you look at the amplifier power necessary to drive these systems, you quickly reach the limits of your typical 15A breaker when pushing them to their limits. A single Maelstrom will soak up 2000-3000W in peaks. You have to consider what is practical in terms of amplifier power and the limits of your typical AC line. I know people upgrading to dedicated 30A breakers for their subwoofer system alone so once you get into this kind of territory, you have to consider all the factors. You also have to be careful to overdesign the system when heavily equalizing sealed systems. You are putting a lot more power into the voice coil and subwoofers are typically only about 1-2% efficient. If you put 1000W into the coil, 980-990W of it is dissipated as heat. That is a LOT of heat for a coil & former to handle. You have to be wise in your application of excessive equalization at high SPL.
