Penn, that wall doesn't look finished, unless that's a dated picture. You can easily knock out a bigger cubbie above the screen for a center and use the lower cubbie for your subs. I can see how you would be outsubbing your 'mains' which look more like rears
I also see why you want a single large driver. A pair of tens or eights, plus a mid and a tweeter should fit easily into 17x25. A 15 would be tight considering wall thickness of the cab but doable.
I made a pair of 24 tall 12x12 3-ways using two vifa 7" woofers. It was not only cheap, but plays pretty loud with good accuracy. Combining smaller drivers is nice sometimes, because of the +6 dB for every two - plus better cone control. On the other hand, there are 15 inch cones out there that play naturally loud anyway, AND go down pretty far. Check madisound also, they've got some good stuff that doesn't happen on PE.
personally I got away from the gigantor mains after having two ported 15s (18x18x36) that largely went unused (used them as B speakers for party time). It's impossible to play them at a normal level. Either that 15 is moving and making ear splitting SPL, or it's not moving and you can hear all sorts of distortion. There are less efficient 15s out there that would work better as a middle ground, otherwise you'll find yourself using two sets of speakers like i did.
The multiple small drivers approach is good because you get plenty headroom and coverage, AND accurate SQ at any amplitude. Your situation won't allow for arrays like you said, but array theory works for two medium drivers as well as it does for 16 little ones. Just don't focus purely on the 15 inch e-penis factor. Genelec and such do it because those speakers will usually see enough juice to keep that big cone in check under normal use.
TundraSQ - glad to hear you found a way out of that situation that makes the both of you happy. Are you using processing for it now or what?