The other day I was demoing a bass for someone and was using an AI EX extension cab and a Focus Series III head. Every time I hit a low E, F, F# or G, the cab would buzz as if there was something in the room responding to that frequency. I used another cab, an Epifani UL112 to make sure there wasn't a problem with the bass or the amp or the cable. With the Epifani those notes were phat and deep as they should be. I then began to thing maybe the speaker in the cab was blown so I turned the cab upside down to inspect it. I discovered that the grill protecting the down firing woofer was pushed in towards the driver. I took the grill off, and bent it so that it no longer was pushed in and that solved the problem. I think that when the grill gets pushed in it contacts the outer edge of the driver which is the soft part that flexes in and out, causing less movement when you need it most i.e. the lowest notes on the bass.
I have two of these cabs and both had the same problem until I adjusted the grill by bending it back into its original shape.
Hopefully this will help someone experiencing the same problem. http://www.jazzstudies.us/images/AI_EX_Cab.png