I don't think you need 15Hz.
People are going to tell you that "accuracy and tightness" are not a function of sealed v. ported design. I don't know. I like my SVS ported box, but it does have a signature sound that isn't quite as dry as some sealed subs I've heard. It's quite powerful enough, and I think I've got more cubic feet than even you do, but my listening habits/needs might be different.
I'm thinking about the sealed, 18"
ED A7S-450 for $850 shipped.
I don't think you can go wrong with any of the other companies mentioned.
If you have multiple listening positions (i.e. you want to share the rumble rather than just rumble solo), I'd consider a couple of good-quality 12" subs rather than having one monster.
There's really no substitute for trying something out in your room.
You not running a sub at all at the moment? Do you know if you have any bad nulls in your room?
