Ok, having reread Blue's original post, I'm confused by the model. Blue, you added 335g per
PR, but the model says 465g. It only changes the overall tuning about 1hz, but still a difference.
I believe the boost in your amp comes from a rumble filter. To model that in WinISD, you set the filter type to Highpass, not PEQ. Hopefully, the manfucturer supplied the Q and Fc for the filter or a chart similar to PE's boost tables for their plate amps, so that you can reverse calculate the Q/Fc needed to get +6dB at 35hz.
Just for fun, using PE's boost chart for the BASH plate amp, I plugged in 1.8/27hz...that's a +5dB boost in the 31-35hz range...most boost the table has. With 240 watts and 335g per
PR, the model doesn't get close to Xmax down to 10hz (limit of WinISD)...max driver excursion is roughly 8mm at 35hz. It also roughly confirms your room measurement by getting much steeper below 24hz...the rumble filter is now combining with the natural rolloff of the driver/
PR...above 24Hz, the driver/
PR are relatively flat so you only "see" the rumble filter.
The rumble filter is likely the source of your woes. With the rolloff that it induces starting around 40hz, you're only capable of 97dB @ 20hz. Take the filter off, and you get 103dB @ 20hz while still staying well under Xmax anywhere above 18hz. The overall SPL curve becomes much flatter as well. My advice would be to find the proper resistor changes necessary to drop that rumble filter to 18ish Hz and close to a .7071 Q. That's pretty much the default for PE's SA240 and BASH-300...I think their Q might be 1.00 which will provide a very mild boost just above 20hz.
Depending on your listening habits, you could even use a traditional power amp with no rumble filter...240 watts will quickly drive you past Xmax below 18hz, but if you don't listen that loud, it won't be a problem. FWIW, I don't run a rumble filter on my ported DVC15...tuning is 17hz so there's plenty of material below tuning that could cause the driver to unload and I do have enough power on hand to overdrive it below tuning. However, our typical listening volume is -10/-15 to reference or around 95dB peaks with all channels engaged. Even at reference level, I've never heard the drive struggle...granted I sit relatively nearfield at 7' to the sub. YMMV.
-Brent