Subwoofer damping

The vents are 4 inch.

Acually I got out a tape and measured the box: 20WX41Lx50H

The box must be a folded horn or bass bin....it is too big for acoustic suspension. The problem with folded horns is the out of band noise from cabinet vibration (two parallel sides along the horn) and issues at sharp angles as you turn corners. You may need to open them up as something may be loose (perhaps glue has dried out and cracked when it was stored or something warped due to humidity/damp).

Bass bins are what would be used in a disco bar...(a type of band pass design)

See this for an explanation

I remember Madisound carrying a very expensive Scan-Speaker woofer with a short coil/long gap geometry. I recall a peak-to-peak x-max of about 10 mm, or 5 mm one-way. I think this was a 12" woofer, but maybe yours is a larger version using the same motor.

Looks like the box volume is about 20 cubic feet.

The "variovent" is not a port; rather, it's a pressure-relief device. The use of variovents actually decreases the very deep bass output and/or increases cone motion relative to a sealed box.

Anyway going back to my over-excursion theory: In a prosound application, the woofers were probably protected against being overdriven by a high-pass filter which filtered out signals below maybe 32 Hz. You see, in a sealed or aperiodic enclosure (which yours is, with those vario-vents) the excursion-limited output at 32 Hz will be about 10 dB higher than the excursion-limited output at 20 Hz. So by protecting the woofers with a high-pass filter, a lot of headroom can be gained.

If the woofer's motor is indeed of the short-coil/long gap ("underhung") topology, that's usually very linear up until the point that x-max is exceeded. Once that short voice coil is excursing beyond its linear range, distortion goes up very very fast. My guess is this is what you heard.

You could increase your excursion-limited maximum output between 30 and 20 Hz by perhaps up to 10 dB by replacing those four variovents with four 4" diameter ports each 5.5" long, but the result would probably sound too boomy unless you could equalize it. If you have a suitable equalizer, that might be the way to go. Otherwise, I suggest some sort of high-pass filter to protect the woofers against over-excursion. Unfortunately, I really don't think an increase in damping material is going to solve your problem.

Duke

I just read Dukes comments and I still don't get it ... a variovent is supposed to allow for a smaller box than the woofer would normally require....but the box itself is already so huge.

Maybe this design is close to an infinte baffle subwoofer with IB type drivers(excursion protected)?