Ok, so I made two (2) cardboard mock-ups using the dimensions of the PB-2000 and they are going to be too large for the space. Even though the wife said it was okay, I made the call that they would be too obtrusive. So I am back to my original question which is add a 2nd FF4 sub or not? I am not confident that I can get the low-end bass I am looking for from the FF4's. I cannot find many reviews on the FF4. I am open to suggestions regarding brand of subs, etc.. As far as dimensions, I can go up to 18" each way. So far, this forum has been the best for me throughout my system upgrade, etc. and I appreciate everyone's input. If more info is needed let me know and I will provide whatever I need to.