So, just to throw in a couple of things here I didn't see mentioned, try adjusting the phase on your subs. That may not be an issue, but it could help a fair amount.
The other is the high port tuning. The box recommendations give a nice boost to the low range of your subs performance. Put in your car and you'll get another boost in your low range. Where and how much depends on the car. Its not that your sub's aren't playing the upper range loudly or cleanly, its that everything else below is so **** loud that by comparison the upper range is week. I like to tune low, not just to get the low range extension, but to even out the response curve so everything sounds clean and even. You will loose some output at your peak notes doing that though.
And the last thing is orientation. This one can have a huge effect. With all else being equal, if you don't have your sub's in the right position to minimize reflections, everything else could be perfect and you'd still have dips in your upper end response. There's a link around here I need to find and post for you, but basically you want the subs as close to the back of the sound chamber to minimize the reflected wave being out of phase and canceling out some of your output. Typically this means in the furthest towards the back of the trunk or SUV as you can get with out affecting excursion or port clearance. The other way is to wall off your sub's so that are literally the furthest to the rear of the sound chamber you can possibly be. This also means pointing the subs to the rear of the car. Thats another ~ foot of reflections you could minimize.
