One final thought, then I'll stop nagging you about the CSS driver. Sorry if I'm irritating.
With a reasonably-sized enclosure, you'll get a lower system Q with a 12" driver than you would with a 15 or 18. I admit I haven't heard any 18" home theater / hi-fi subs, so my assumptions about Q could be unfounded. Indeed, Tom V. of Power Sound Audio tells me that worrying about Q is no longer in vogue and hasn't been for several years. But I recall years and years ago when I was shopping for car subwoofers, I preferred the tight response of my JL Audio 10's to the 12's of the same series. The 10's just sounded more musical, and I think it's because they had a lower Q than the 12's in my trunk.
That experience was a consideration when I was shopping for my own DIY project. I read somewhere that a Q of 0.5 was the magic number. Lower would be too damped, and higher would result in less control. Thus was born
my list of potentials.
The CSS in a 3 cu. ft. box has a Q around 0.5. Any larger subs I've modeled in a similar sized enclosure have had a Q of 0.8 or higher, as well as a steeper subsonic rolloff. Now, I don't doubt a Q of 0.8 would be perfectly acceptable for movie effects and for music not as demanding of obsessive control. But for a tight, musical response without needing a 6ft³ or larger box, the CSS was my huckleberry. Please know that I can't say whether a different choice wouldn't have been as musical. All I can tell you with certainty is that the CSS has delivered, and that my assumptions, as far as I can tell, have at the very least not been counterproductive.
On the other hand, the Dayton 18, the SB Acoustics 15, and all the others that have been mentioned in this thread must be doing something right to warrant their recommendations. I'm probably putting unreasonable emphasis on a feature that doesn't have much benefit in the real world. If that's the case, I truly apologize, and I hope someone more experienced teaches me something.
I'm also thinking about subsonic rolloff. In your first post, you mentioned you wanted solid single-digit Hz response. Is that still your goal? Did you ever download WinISD and model the drivers you're considering?