Dayton 15" IB in small, sealed box?

Assuming that you mean Dayton IB385-8; according to WinISD you can put it in a 120 liter box and get a Q of 1.0. Then add a 39 Hz 1st-order highpass filter (capacitor) and you'll have a perfect 3rd-order Butterworth system response that is -3 dB at 39 Hz.