Ya dude, the bigger the sub the better. I fugured out years ago with car audio that 1 twelve will out perform two 10's all day long and one 15 kicks two 12's as long as they are approx the same quality driver......there's no replacement for displacement!!!!! The mfw over at av123 is a great sub for 700.00, or get a used pc-ultra. An extramely high quality 12 is the absolute minimum!!!!
I thought the performanace size ratio was something such as 3 X 10", 2 X 12", 1 X 15", .50 X 18", and so on. I agree with what others have said. Having at least a 12" subwoofer would be best because otherwise you will need more than one 10" sub.
I currently have 4 ultras, and I would near do any thing for two 18"s. So if I were you I'd try and save like hell for an 18" (nothin wrong with used brother) and add another one down the road. I would NOT wast time or money on a 10" anything (unless it says sunfire.......don't know how they do it!!!
I have 4 of the 12" ultras, the 13's should out perform my 12's.....although I do not know how much. From what I hear 18's play much lower with less decompression, and a lower tuned inclosure means more output down low and less up high. Which is what hometheater is all about. Plus you can build sonosubs pretty reasonable. I've not completed my first 18 yet so I'm just going by what I read at the shack. My plan is to put the 4 ultras upfront and run the 18" in the rear. Sounds like most guys that built there own subs eventually sell the commercial subs, so I assume thats the way to go..... I will eventually find out :T rich