yeah, you probably don't want the passive sub. most subs that have built in amps have built in crossovers too. so they are really easy to integrate into your existing system. many subs also have inputs for all 5 channels of surround sound, if you want to use it that sort of environment too.
anyway, normally you would hook up your source, or preamp outs to the sub. then take the sub's outputs and connect them to the loudspeaker's amps. pretty easy. if you get a passive sub that doesn't come with a controller, you're gonna end up with a much more complex system.
also... you didn't mention what loudspeakers you're using. you probably don't want the biggest subs for tiny monitors... it'll just not be very balanced. but if you have modestly large speakers to big ass speakers, i would recommend the Bag End Infrasub 18"..... these things are badass! they have a measured bass extension down to 8hz!!!!!! and no, they ain't lying on their specs. Bag End is well known in the pro audio world, and i have no doubt they are plenty serious with this amp. no, you cannot hear 8hz, but you can feel it!!! if i had a home theater, these would be my dream subs... even if i had a choice of more expensive models. the infra sub retails for about $1500, but i saw it on ebay for $800-900 before.
if you want even more juice than the infrasub, bag end sells professional systems that include double 18" woofers + custom controller (their controller is required for 8hz operation.) but that's big bucks. supply your own amp too.
...for powered systems, i still recommend pro recording hardware above all else. Mackie and Genelec both introduced dedicated subs for their powered monitors... but prices range from $1500 for low end mackies and genelecs all the way to $8000+ for the bigger genelec subs.
personally, i use the NHTPro B-20 dual-sub system ($2000.) but i probably wouldn't recommend them for any other speakers than the A-20's. they don't shake the room like many subs... and don't sound as deep. but they match the a-20's sound very well and are very accurate. but probably not very impressive in an audiophile system. though if you want accurate and non-exaggerated bass, you could try them out. plus they're stereo (can be switched mono via remote--also has bypass on remote,) which is interesting. theoretically stereo-ness shouldn't matter in bass frequencies... but the subs do produce some sound in the higher-range. so.... i dunno. i have them mounted on 26" stands just like stereo monitors.... sounds great to me.
