As much as I love my subs, I think this is an area where a long game approach is going to suit you better.
Subs and LF being as finicky as they are towards room volume, I can't help but wonder if you wouldn't be better off waiting a little.
I know, I know... Its a horrific thought. But here's the kicker... if you buy a small sub now to hold you over, thats gonna be, what, $500? Buy your towers so you get close to full range now, and be ready to buy the right sub for the new home! If the right sub is 2 15" rythmiks, what are you gonna do with a 10" speedwoofer?
Ya its a good product by all accounts, but if you need to fill >5000'3, even 4 of those won't necessarily make you happy.
Anyway, as much as we all love to help spend other peoples money... I want to spend yours where its gonna matter the most!
I'd hate for you to have to sacrifice great bass later for something that won't anger your apartment neighbors now.