The crossover is no problem, as it is done in the Denon 4806 receiver... subwoofer preamp level out.
The box will definitely be a sealed box, no ports or vents, for smoother response.
I can calculate what the box size should be.
I just sort of cannot find a "home" raw woofer I like, and can find raw car woofers I like.
The woofer I have the 12" Infinity is sort of not terrible, but it could be a LOT better.
The actual speaker in the Infinity sub box is a chunk of junk. Ditto the Klipsch 5.1 computer speaker broken down trash box.
The retail subs I see have not got anything better anyway in the price range I can afford.
(A nice Martin-Logan $3,000 or a B&W $3,000 would be ok... but out of the money range I am willing to throw at the problem.)
(am I in a nasty mood or what?)
So far the Canton's do well, they just need the last low freq. from 40hz down to 22hz.. enough to give the bass a bit of feel.