Wow to much output on the 21" for a port. Would that mean that sealed it wouldn't have more low end than the Maelstrom 18" ported? Sorry if this is a lame question.
Also Kevin do you think this will be available by August?
Three things you deal with. Remember Hoffman's Iron Law. Box size, efficiency and bandwidth are all related. Change one, you have to compensate with the others.
Assuming everything else is the same, the box size scales with the surface area of the driver. If you keep the box size the same and increase the surface area of the driver, you pay for it in bandwidth (everything else remaining the same). That means if all other parameters are the same, as you increase the cone size by 30% you need to increase the box size by that same proportion in order to maintain the same F3.
Many people automatically think a bigger driver will go deeper. That isn't the case. A bigger driver gives you more potential output (given the other variables stay the same) but it won't go deeper (assuming you hold all other variables fixed). You need to either equalize it or design the system with a large enough box to get good bandwidth AND good extension.
What you will find with the Exodus driver lineup is that we have chosen parameters that allow close to the same bandwidth, as you scale driver & box size. This is a little bit of a simplification and not strictly true but I tend to pick parameters based upon a given modeled response. Bigger drivers just get bigger boxes on average. I bend the rules slightly with the Shiva-X because it is one of the smaller drivers. Try to push a 12" to deliver really deep bandwidth and you will quickly find it running out of excursion. For that reason, I tend to like to run the ported alignments of the Shiva a little higher than I would a Maelstrom. I'd tune a Shiva @ 20-22Hz, and the Maelstrom @ 16-18Hz. Why? Because the Maelstrom has a lot more potential output and is less likely to reach it's limits. Another rule of thumb to remember is that you need about x4 the amount of air movement capability for every octave you drop given the same SPL. Smaller drivers just don't have the same headroom as does a big 18" or 21" driver and when your pushing for really deep extension at high SPLs, you need every bit of driver you can afford.
Oh.... horns follow different rules. Horn loaded systems are not within the scope of these rules and for most 1st/2nd octave applications, they are not a viable choice. Horns are awesome but they are devices that have a very narrow bandwidth. True subwoofer horns are rare because of the size needed. Danley has done some really cool things with horn loading and Tom's much more of an authority than I on the subject. For most DIYers horns are an extremely complex build. Wiggins did a horn design with the original Tempest and while it was capable of scary dynamics, it was bandwidth limited. It used the sealed box behavior of the driver for the very bottom part of it's bandwidth so it had limitations based upon the driver on the bottom. Up higher, it was capable huge dynamics but it just didn't sound right in a typical living space to me. Bruce Edgar makes a big sub using a pro-sound driver that sounds great. It is bandwidth limited even though it is the size of a refrigerator. It wouldn't be a great HT sub necessarily.
Like any engineering exercise, your playing with trade-offs. You don't get a free lunch. You just have to figure out which lunch you can afford and which one is most likely to fill your belly. :T
I'm projecting sometime in August. Right now that is a promise from the factory. They tend to be optimistic on their delivery schedule so it could always slide. I'll have firmer dates in about a month.
Kevin Haskins
Exodus Audio
It will be a dual 2-ohm design like the rest of the lineup. It will be a the same sort of cone, dustcap design as the rest of the lineup. It will have the same AlCu inductance configuration that is working really well based upon the Klippel data with power, stroke and frequency. Overall, there is no reason to make radical changes to the motor and there is some economy of scale in keeping it common to both drivers. 
