I am doing a bit of research and trying to figure out what if any gains I can get from say 16...18Hz - 80Hz by moving away from the dual Ultimax 18" flat packs (build thread here @ AVS). MiniDSP 2x4HD is used, and the receiver is setup with no subwoofer (integration, xover etc. done with the DSP). The mains are the HSU Research bookshelves powered by a Behringer NU1000DSP. The room, which is shared with the kitchen, is approximately 7ft8in tall, 23ft deep and 20ft5in wide, for a volume of roughly 3600 cu ft / 102 cu m.
I do not think a ported design is possible due to the limitations of the external dimensions. The flat packs are 23" H x 20" W x 20.75" D, or 5.52 cu ft, and I can go up to 25" H x 21" W x 21.75" D, or 6.6 cu ft.
First, I was looking at building new ported boxes for the current subs, but I cannot find designs that fit within 6.6 cu ft externally. PRs also get expensive, but it might be worth it (around $900 shipped for 4x). I was eyeing the SI SHS-24 which is on sale right now, but the cabinet just cannot be made small enough. So, I think I have to stick with 21" drivers. The IPALs would not be a great fit for sealed enclosures I believe, so I would have to look for other 21's.
Am I on the right track here? Using multiple smaller (potentially ported) drivers would also be an option perhaps, but stepping down in overall area doesn't seem to be that efficient.
Edit: To give you an idea of what the room looks like, this is the living room, picture taken from the kitchen.