Say Watts ????

With regards to building subs the easiest design to build is a bass reflex design, all you have to do is choose a driver, then use software like for example WinISD to model the port length and port size and include bracing inside the enclosure and you can come up with a pretty damn good reflex sub. B&C drivers are excllent so have a look here :

There you have drivers and designs for enclosures that BB&C reccomend for their drivers. As for amplifiers i'd say the Soundgear Saturn amps are unproven as of yet and i've not seen anything from any users which says whether they're good or not. I have QSC RMX4050HD amps so maybe on my subs and they sound excellent, you also have to remember that just because an amp looks very powerful doesn;t mean it is. A lot of amps power ratings are rated in burst not continuous average power, and then even then many amps don't have power supplies good enough to sustain sub bass. QSC would be a better option to go for because QSC are one of the best amp manufacturers in the world i've always had QSC and they've been brilliant and CAN output the rated power continuously.

Just to point out, the decision of active vs passive crossover is wholly separate from which box the components live in - you can just as easily bi-amp a combined mid high unit as you would separate boxes.