My 20-39 PCi does just fine in a room similar to yours in size. BTW, that is a great receiver...
If I were to do it again, I'd pony up just a bit more and go with a 25-31 PC-Plus tuned to 20hz. You'd save about $50 in comparison to the 20-39 PC-Plus, $125 cheaper than the box version and it would be a bit smaller than the 20-39. Not drastically smaller, but 8in. shorter, which might help in location placement.
The PB12-Plus would be just as nice, but I found their box subs to be a little too big, eats up too much room horizontally for my taste... -TD