I can't argue with it, two of the PB10s would give you flat response to 20Hz while providing very high levels of output. I don't know that you could do better for the price than a pair. I have one and it's quite an impressive piece to say the least.
I was told that the driver on the PB10 is closer to the PC_Ultra design and more suited for a "Music" setup than the PC+ driver. Man...by the time I bought and shipped two PB10's and purchased a 30+ foot sub cable I'd be near the cost of an Ultra. Damn...this is frustrating!
Actually I did. I sent an e-mail asking which would be better for a "Music" setup, the 25-31+ or the 20-39+ and listed all my equipment and room specs. Ron would not commit on one over the other. He said either would do nicely and mentioned I might should consider upgrading to the PC_Ultra.
With a room that size, I'd go with one of the cylinders or a PB-12. The PB-10 does fine in my 19x20 room with high vaulted ceilings. I've heard a 25-31PCi in a room even larger than yours and it still pumped out an astounding level of bass that sounded great with music.
The differences between the two are miniscule, especially for music. The amount of content between 19 and 24 Hz (where the larger sub will perform better) is very little unless its pipe organs or genres with synth bass (dance, some rap). I've had them side by side doing an A/B. Pusing to the limit, the 20-39 does bottem before the 25-31 (still very loud. I think the extra output outweighs the greater extension (plus the 25-31 is smaller and cheaper) but thats just me.
Whats WAY more important for your application is room treatments and/or BFD equlization. The mangling your room will do to your deep bass is orders of magnetude more audible that the difference between these 2 subs.