If the issue is floorspace, you are not going to find a current 12" sub that will take less of it.

Since you want to go box sub, the new PB12-Ultra is probably what you want since you like SVS. Ofcourse I like the VTF-3MK2. Both of those subs will take up more floorspace than your cylinder did, but will fill the 18x18 room easily.