Its the box or tuning on amp or hu..that box is small for a xfl if yr at ur max space try trading it for a xfl 12 ...the box is probably 2.6-7 after sub amd port disp which is horrbly small and could be a big part of the problem ..the xfl is a very solid sub and the ppi bk1300 is a very nice match
And sub up port back works best in those cars. I f you can do the math and get close to 4cubs after disp. And around 14-17" of port per cube u should be very happy