port tuning

My guess is the two subs together mounted to the box would displace about .15 cu ft.  You might try to search online through some of the new line of xplod subs and see if there is a displacement value shown on any of them.  But you should be pretty close using the total of .15 cu ft.

So if you're modifying the box to have 4 cu ft left over after all is put into it, make the port length 14.33" long to get 40 Hz.  10.25" long to get 45 Hz, which is just a tad louder SPL. Or use any length in between.   And keep the 4"x 12" slot opening, which doesn't change.