The problem stems from the Peerless CSX 10 drivers I'm using in my bi-amp mains. These drivers have an exceptional sound to them until I push them a little. First, a little info on their use...

A pair of CSX 10's for each channel(L/R), in an enclosure equalling 180L in volume, tuned to 30Hz via 5"id x 9"lg port, wired in parallel for a net imp. of 4ohms, driven by a Numark Dimension 3 amp in 2ch mode w/onboard active x-over set at about 100-125Hz. Amp is rated at 475wts rms/ch at 4ohms and is 2ohm stable. Amp has clip indicators and multi segment vu meters for monitoring.

Now, Here's the deal. I hear, durring loud passages of music(rock), what would initially apear to be a sound simular to a VC bottoming out or a VC that is rubbing on the pole or mag housing. It doesn't happen all the time, with all types of music, or below certain frequencies. It's not as harsh as a VC bottoming(I have LOTS of exp. with this) and is more instantanious than a VC rubbing on the motor. It can and does happen mostly at the beginning of a bass note(as opposed to the peak of said bass note) but will also occur in greater quantity and at lower power if the x-over is raised towardes the 200Hz area. It is not a function of the box as it will occur in free air with the same power and content variables applied. I've measured the input power to the drivers as wired and the input is not exceeding the manufacturers specs. There is no sign that overheating is occurring either at the amp or the speaker and problem occurs whether listening cold or warmed up.

I've had a relative with more practical experience than I listen and he said it sounded to him like a sonic boom from a vented spider. While the CSX 10 doesn't have a vented spider, it does have a vented cone(8 holes around the parameter) above the spider and below the dust cap. Now I can close off the holes to test the theory, but if it proves to be true, what would be the long term fix without affecting heat dissipation? I'm considering foam over the holes instead of tape so air is slowed thru the holes but not stopped. It would also be "physically" possible to drill out the center of the motor for additional venting but I'm worried about chips and shavings in the VC area.

Are there any other possibilities I may be missing?