The vents are actually a plus for bass response & act as a sort of bass port, similar to a vented speaker enclosure. Also, if you seal them up the pressure from the woofers have nowhere to go & will cause all kinds of things to vibrate inside. I did this experiment with both a 777 & M90 & it actually killed the bass & caused all kinds of rattles that were impossible to control effectively...
DON'T BLOCK THE VENTS or your hard work will be wasted....(caps are for emphaisis, not yelling, sorry)
Any thoughts on how you are going to AMP those cans? I'm guessing a single 931/975 amp won't cut it for power....T-amp or dual stock amp setup??
Also, you may want to look into dual power blocks for added amps...Norm may know what to try...
Cpl