I have looked at this question too. My personal conclusion is that it you're looking for raw output, two PB1-ISD's might be better. But if you're looking for low freq extension, one PB2+ is probably better. The reason why is that even though driver area is the same, the PB1-ISD low freq extension is still limited to 20 - 22 Hz, even though you have two of them. They just play louder. Don't go any deeper. But for the PB2+, the two drivers are equalized together in the box for low freq extension too. But then again, the PB2+ can be "tuned" to different low freqs (and outputs) with those muffler things. So ultimately the single PB2+ also gives you more flexibility too. But, ... 2 PB1-ISD's are easier on your back!