I agree that it's impossible to have too much sub if it's calibrated correctly. The only thing you could do is buy a sub so large that you have extra headroom you'll never use, but with the size of your room and the opening in it, the PB12-Plus is actually a perfect fit. You'll be very well set for most all listening levels.
I have a PC-Plus that's lasted for 5 years and it's still the best investment I've made in my system.