Audio Systems: Home Theater Sub, spot seat, tower speakers

Question
Hello, I'm wondering how I would go about building a decent sounding enclosure for a subwoofer I just picked up.  It is a 15" 8ohm 150watt RMS 300watt peak sub. I guess I'll begin by expaling my setup. My speaker list is as follows" 2x 150 watt Fisher tower speakers, 12" 4" 1", 2x Technics 150 watt tower speakers 12" 4" .5", 2x Radioshack PA speakers 15" and 3 piezo horn tweeters, 1x 15" subwoofer, because I never crank my receiver I have the fishers run as rear surround, PA's run as fronts, and Technics run in parrallel for center channel, my reciever is rated to run all 5 channels at 100watts @ 6hohms, anyway... I want to have this new 15" sub I picked up as a dedicated subwoofer, I want to have it setup as a down-firing sub with possibly a passive sub, I know that in order for the passive to work correctly I need to have it 180 out of phase with the primary sub, I want to have the enclosure ported... so, to sum it all up
15" sub
150watt RMS 300watt peak
8ohm
want: ported enclosure w/ possible passive sub
any ideas?  

Answer
point blank:  LESS IS MORE!!! it sounds like you are going nuts here thinking adding more and more and more and more speakers is going to improve something.  Having all these different larger speakers is going to make a giant mess of the bass if it's not crossed over properly.  you've got 2 different sets of 12" speakers and then 15s and now you want to add another different 15"??? And you have 2 tower speakers as your center channel???????  the whole purpose of the center channel is to be 1 and only 1 source to draw your attention to on screen vocals and effects.  having 2 will just blur that all over from every seat other than the sweet spot seat.  Then out of all the speakers you own, you've got probably the worst sounding set as your primary ones (main front speakers).  the radioshack PA speakers? your front speakers carry the majority of music and off screen vocals and should be the clearest speakers you own and should be closely matched to your center channel speaker as possible.  I'd love to help but I dont even know where to start here.....
bass will get canceled out and comb filtered with a setup like this.  you need to pick 1 sub or a matching set of subs to be where the bass is produced.  ALL other speakers need to be highpassed so that they dont make bass that will interfere with them or you'll just end up with less bass and wasted energy.