I've gone through quite a few boxes with this sub, and surprisingly it IS loudest with the 27hz box with the smaller port to PB. I get 2-5 dollar mdf, lot's of trial and error.
I only burped a 146 with my 39hz box peaking of course at 40hz. the 34hz box only did 146.7 at 35hz. and this, the 27hz box is much smaller. It seems to allow a lot of air back into the cab because even with the smaller port the air movement is crazy. It also could be thefact that this sub is designed to take lower frequencies much more givingly with the 4" peaktopeak. Lots of play and little to no clipping on lows out of the setup.