mixing sub sizes

If I hook up a 10" and a 12" on one channel and repeat on the other channel, that would be balanced, right?

Never, ever, ever mix driver sizes except for this way or unless you have the necessary background to do it right. Some people will say that you can do it just fine and see I have done it myself and it works great.......until 4 months from now (looked into my crystal ball). If you are going to mix driver sizes this will lead to cancellation, it will lead to unbalancing an amp, it will lead to headaches. What needs to be done is the range of frequencies restricted to each type of driver, this means that a lowpass crossover would operate on the largest subs and be crossed over at say 70hz. Next the 10" subs would take over from 70hz - 120hz, thus a bandpass crossover is needed + a second amp, next a hp crossover to power the balance of the front and rear speakers. A 10" sub can play the same low frequencies that a 15" sub can, it just cannot play them with the same authority. Thus the subs will be playing overlapping frequencies. This will lead to the smaller drivers usually seeing all sorts of distortion caused by a clipping amp because you are trying to get as much bottom end as you can and the larger drivers will be overpowering and masking the distorted output of the smaller drivers.

If you are trying to get loud, multiple small drivers is a great way to do this. With the doubling of surface area it will definitely play louder and with much more authority on the lower bass notes. Your best bet is to add another set of the same subs with the proper box and proper amp.

my opinion is right in there with Dyohn's, and forbidden's. (sry didn't mean anything by the order i put them in. haha)