Are you saying there's no sealant between the driver or XO and mounting surface? If so, I'd be very surprised if there's no leaks and based on your various observations WRT vent lengths, various XO woes, etc., it would explain them all except maybe some portion of the high intensity beaming which can be dealt with by adding a foam insert, but that comes under the heading of fine tuning.

AFA how big a leak it takes to basically delete the vast majority of box loading it's as little as air leakage around the mounting bolts and T-nut threads, but an air leak around the driver ~completely negates any box loading.

GM