I feel that most of the commercially sold subs out there are made improperly and are using cheap components. You can save so much and build amazing quality by selecting the components individually and not buying something that a company can sell anything of (cause of their name brand). An example would be Hafler... they sell a powered sub that is downfired and has an interesting ported enclosure. While it might be efficient and loud, it is designed not well to create a flat response, and brings too much of the room's acoustics into play. Alot of the subs out there are ported, and drop off response below the tuning frequency.
I'm just speaking critically, but I've made some poor mixes with some bad subs. Any sub is better than no sub though..
I like your idea about using a 5.1 receiver to power your system. There's no problem running multiple speakers off a consumer receiver like that, and I'ld say go for it, since your listeners will be using the same! As for running a sub on it... many 5.1 newer receivers may have started implementing Bass Management, which cuts the lower frequencies off the amp's channels and sends them to the sub, summed together with the LFE channel. Since there aren't sub amps on 5.1 receivers, you will need to make sure that the main channels can output the full range. You would need to also use the "direct input" RCA's on the back of the unit, and feed only a lowpass'd signal to the channels that the sub will be using. You still can't get away from using an extra active crossover if you want to run the sub as passive.
Look at PartsExpress.com and take a look at their subwoofer kits. They come with the box (MDF material), amp, and sub. They are much higher quality than so many of the powered studio subs out there! And they sound great and can be multi purpose like when listening to DVD movies. Boooooomm!
If you wanna build the sub cabinet and prevent from having to buy a premade one (AND THE HEAVY SHIPPING) it really pays off to build one. They're simple when following the subwoofer manufacturer's recommendations on airspace for that sub.