I had one PB12-Plus/2 in the front corner of my very symmetrical room for one night... added the second one the next night in the opposite front corner. There was no difference in response measurements. Because is was symmetrical and both equal distance in each front corner with the listening position centered, the only thing a second sub did was give me more impact and SPL. It was a noticeable difference.
The key to getting a second sub to help smooth out your response is to be able to experiment with multiple locations. Once I got to experimenting I began to see how it could help me smooth out my response. I ended up placing my second sub in the rear of the room, about equal distance from the listening position, both in phase... and twala! Since then I've built a sub for the rear and placed the Plus/2 back up front where it was. 2 up front and 1 in the rear. Works beautifully!
