I should have told everyone that the sub is a Polk PSW505 (rear-slotted vent, with 12" driver, and DSP circuitry). I'm playing Bach & Widor organ music. I've taken off the sub's grill so I can watch the diaphram move, and put my hand in front of the rear slotted port to feel air movement.

I can barely see the diaphram move, and there is no air coming out of the rear-slotted vent. Volume is set at 12 o'clock, and the low pass filter to 60 degrees.

Eric G.