ok...so take this horrible, horrible graph to better explain what I'm guessing:
1.) You run a single 8" sub (the lower blue line)
-it doesn't hit as low
-it seems to be loudest ~80Hz
-it doesn't hit low or loud for rap music, but with your "Peter and the Wolf" CD, the sub really makes the frequency range of an oboe roar
2.) You run a single 10" sub (the lower green line)
-it hits lower than the 8"
-it seems to be loudest ~45 to 50Hz
-it really doesn't make an oboe loud (and sort of muffles it out), but it really makes your techno bass kick
3.) You run 2x8's or 4x8's (the raised blue line)
-yes...you have MORE surface area, so the subs push MORE air, thus it sounds LOUDER
-but by increasing the number of subs, I think you do, in fact, increase bass on the lower end, but you also also increase bass where the 8" sub hits well (~80Hz)
-since the 80Hz peak increased as well as the 45-50Hz bass, all you really hear is the 80Hz sound
-so in essence, you're still hearing the oboe with more 8's...except now it's just louder because it has more surface area. It doesn't necessarily mean more 8's will hit lower than a couple 10's.
Does this make sense?
Again...I'm not sure how this works, but vis a vis...this makes sense to me.
So what I'm trying to say is, stick to the 10's