Help with AVI SL-250 10 sub

I just built a box for an AVI SL-250.  The enclosure is 1.15 cu ft with the recommended 4" diameter x 12" port but it sound lousy.  I know this speaker is good cause I've heard them pound in bandpass boxes (I mean from blocks away powered by 100W alpine duo B amp).  My system sounds lousy on the kicks and isnt very audible in the lows.  I noticed that when I pop the trunk the performance goes way up and the sub starts to sound as it should.  I can turn the volume way up and the sub doesnt begin to distort at all.  Does anyone have any suggestions?

Here is the spec sheet for the sub

http://avisound.com/pdf_files/SL-250-SP.pdf

thanks.

I think your problem is your tuning frequency.  45 Hz is very high to tune.  Modeling it up it doesn't look like you get much low frequency extention.  In fact it looks like the response of a midbass driver.  You get a steep rolloff at 50 Hz.

To get a semi flat response from it, try lengthening your port to 22 inches.  This will tune it down at 35 Hz instead, which should keep you flat down to 30 Hz.  Just make sure you have a subsonic filter.  That sub doesn't have much throw.  It will be very easy to unload it, which will destroy it.