I use Rythmik subs, so I'll second that recommendation. SVS also makes a good value subwoofer at about $1K/pair (and a pair is better than a single sub IMO).
As to the best way to cross the subs, I'm going to go off script here and recommend a pre-pro with Audyssey. I've tried a lot of different approaches using pre-amps from ARC and Joule (among others) as well as some pretty expensive high-end x-overs, including Marchand. My Onkyo pre-pro is IMO far better sounding than any of the high-end set-ups. You may be stripped of your audiophile club card if you follow this advice, but I think it's the best way to go.
YMMV
As to the best way to cross the subs, I'm going to go off script here and recommend a pre-pro with Audyssey. I've tried a lot of different approaches using pre-amps from ARC and Joule (among others) as well as some pretty expensive high-end x-overs, including Marchand. My Onkyo pre-pro is IMO far better sounding than any of the high-end set-ups. You may be stripped of your audiophile club card if you follow this advice, but I think it's the best way to go.
YMMV