I have a feeling my sub box is to small for the new speaker I have. I brought and installed a new B&W FST Kevlar unit to install into my 5.1 Home cinema set-up. The sub box, which is active and contains all the relevant power supply, amp and components is about 350mm deep, 300mm high, but only 130mm wide. The trouble is, the driver is installed on the left side (no other way), so the distance from the rear of the driver and the other side of the box behind it is probably only about 4cm. The box is ported by a 3 inch hole at the front of the box, but I believe the air flow behind the driver is being restricted due to lack of space. I reckon, due to the short distance, the air pushed out of the back of the driver is rebounding and being pushed right back on the driver, causing it to flex and vibrate incorrectly. What do you guys reckon? Time for a bigger box..........?