For ported then pb2000/monolith 10" or 12"
Try and get best sub you can, or duals if possible.
The svs cylinders don't take much space up however svs are now expensive, I've had several going back to 2007. They're now no longer the bargain subs they used to be. The sb4000 is almost double the price of my sbu13 and its the same sub. It's good but not worth £2000. The monoprice 13" are cheaper than sb4000,.and with limited use 13" outperform the sb13 for movies.
Svs used to be like Monoprice , internet direct.
I'm running a three sub setup 7.3.4, with two monolith 13" thx ultra and the sbu13 (actually the older version of this sb4000 but pretty much the same) not in situ yet just plonked in the room and connected up.
I'd look at the two monoprice subs that are 15% discount
I would like getting two subs but I don't have the space. I only have the right side of the tv.
Other side is occupied by my Mrs and mum's stuff and the made it clear that I can't move that. Lol.
I'll try to get a good one but if it's not the best I'm not going to complain.
I'll keep exploring.
The xtz does look the most promising but I wish there was no tax and shipping charges.