Ian,
I'm sure the experts will chime in with all kinds of facts/figures and there are a lot of math/physics that go into speakers, not to mention the entire audio chain, however this is just my opinion....

I've owned all kinds of different brands/sizes of speakers just like viking since getting in car audio in the 80's, and multiple speakers can sound great, heck, one of the best sounding boxes I ever owned held 3 late80's kicker competition 8" subs. they were very tight and controlled, and in the world of audio competitions, they performed very well since they were accurate... very fast sound. that being said, ANY truck that pulled up next to me running a single 12 sounded deeper, louder and had that bass that hit you in the gut. something that any 8" sub i've ever owned just won't do. my opinion and experience over the years has shown me that any sub with a good box and good power will sound good, but to get that low, "hit you in the gut" bass that you get at the clubs, you need larger drivers. it just feels that a 12 will play lower harder than a 10, and it doesn't matter how many 10's, a 12 feels like it will reproduce the lower frequencies that much better and louder. I get all the talk about surface area and moving air, but just because you add surface area, each sub still only plays a certain frequency range efficently.


either way you go, I'm sure you're results will be good, however ask me for my opinion, and I'll tell anyone put a good single 12 in any vehicle for the best results.. heck, even at home, I'm using a velodyne 1210, which uses a powered 10", but it's coupled to a passive 12" to get the lower frequencies covered

good luck and be sure and post pics of the remodel....