Currently I have Boston Acoustics VR2's for my front speakers with a Boston PV500 10" sub. I'm a little unhappy with my PV500's performance. It's only rated to about 35Hz. I would like a sub that plays a little lower and louder. I've seen a lot of good reviews on the Earthquake MKIV10 sub. It has an active 10" driver with a passive 10" radiator. I read one review that said it output 100db at 20Hz with less than 2% distortion. Sounds pretty good to me.
My question is this: When you run 2 subs, do they need to be the same type of sub or can they be completely different, as my PV500 and the Earthquake would be? Also, when you use 2 subs, does one need to be out of phase with the other or should they be the same?