Are they broken in? Because if not you'd probably loose a bit. Any fiberglass? 45 degree angles in the enclosure? How much port area do you have too? These are all factors that will affect your overall score. Sealing off the box from the rear is always good measure for increased pressure in the cabin, since now all the pressure will be in the front of the vehicle.
Seal it!!! This should solve your problem. You should with ease be able to hit 148+.