couple of things.
Those drivers need about 200 to 300 hours to break-in. I would wait for the break-in before trying to "fix" something.
second point, is that driver is really design for back-load horn. Even Fostex says so, and even though they provide the 45 liter box example it really will never do much in such a box. atleast not without a correction circuit. Then this circuit will eat up the efficiency you need with 9 Watts, cause it could brink you down to a few dB.
My best advice is after the breakin if you still don't like it move to the other cabinet Fostex recommends.

good luck
paba