Hi Sweep,
Like the responses above...the answer sort of depends on which models you are considering. For example, the PB12-isd sort of bridges the small gap between the 20-39PCi and the 25-31PCi as fas as extension and clean output capability goes. The larger 16-46PCi would extend much deeper than the PB12-isd...but you'll lose some headroom(maximum clean output capability) >20hz too.
Feel free to contact me at SVS techsupport if you have any other questions about our products.
Tom V.
SVS