ported box design tc1000s

I have a 2001 Dodge Ram 1500 extended cab, I took out the back seat and will be building a box in that space. I bought 2 15" TC1000s that I want to put in a ported box in the back. I talked to the folks at TC Sounds and they suggested a 10cuft box with 4 4" ports,each 6" long. This would tune the box to about 28.3Hz. I've try modeling this in WinISD and I get drastically different results. I reentered the parameters over and over again, but I still get a significant difference. When I put in 4 4" ports WinISD tunes the box to 38Hz and the plot itself looks horrible. Obviously between the two I'd go with what the factory tells me but this is a pretty substantial discrepancy. Anyone have any thoughts on the matter? Has anyone modeled the 15" TC1000 with any success? Thanks for the help.

I suggest you change your chart to show only +12 db and -12 db.  This will show more clearly how much peak there is in the yellow line at +4 db at 38 Hz.  The white response is wayyy too peaky for SQ but might make a good SPL contender.  That is one huge box for a car.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.