The Maelstrom 18 is the only one I know of and I'm not even sure if it's shipping yet, the Tumult 18" has a way to go evedently and the assualt series is still in wait.
If you are referring to Ascendant Audio's assault series it isn't going to happen at all. I contacted them awhile ago and recieved a response from Dale stating that they pulled the plug on the assualt series altogether.
"That's too bad. I wonder if manufacturers are steering away from 18s because the recent 15 inchers can move so much air."
I am not sure. What really struck me as odd was in Dale's email he stated that the assassin line was better than they expected so they weren't persuing the assault series. I'll see if I can find the email and give you the exact wording he used. What is really strange is there isn't even a 15" assassin they stop at 12". I was really hoping for a 18" for around $200.00 give or take.
It seems that ever since the old Ascendent drivers left the market, the really good new drivers are eternally "just around the corner". The Assassin 12 looks like a great deal (at the $75 pricing) for a moderate subwoofer and I can only imagine that a 15 or 18 inch version would have been an even better bargain.
There's talk of at least 2 new woofers being developed over on the IB Cult forum. One is an 18 with a reported 35mm Xmax - almost 8 liters of displacement. But its being designed specifically for IB application, so it requires a huge enclosure. And presently its still vaporware.
On a positive note, Parts Express has just released the 15 inch versions of their RS subs. Not a low-end SPL monster, but should be a very nice woofer.
I put this list together last summer, of the bigger driver contenders at the time, mostly based on info from the forums, and I'm not sure how many are still active in the market. For a while it looked like there was going to be some serious competition, but it seems things have faded out a bit. Either way, here's the ones I had, and if anyone has any additions I'd love to see them. Actually there used to be a page (I think one of the manufacturers) who compared the critical specs and displacement of all the drivers too, that would be handy to have updated.
The Assault series will not be offered. The reason is that the Assassin series was able to do what the Assault series was going to do and for a lot less money.
Dale Kuypers Ascendant Audio"
There is the email I recieved when I asked about the assault series. His statement might be true if they had a 15" and 18" assassin.
Elemental Design is bringing out an 18" driver. They produce a very nice quality driver, and appear to be moving from strictly car audio into the HT end as well. As a direct internet retailer, their prices are excellent.