Welcome to the forum jwright.
Sounds like you have had a rough start.
Axiom will take care of any in transit speaker dammage, including dents nicks etc. You've already experienced their excellent customer service.
Those brackets are not meant to be secured only at the center. Look at the pictures in the mounting instructions. There are screw mounting holes at 3 places on the main part and two on each arm of the T. Put a few more screws in and you will be just fine. I only used two for the main part and one on each arm and they are plenty secure (though in my case all screws are into solid wood).
The ep500 should be able to rattle you plenty in a big room. How did you calibrate spl levels on your speakers and sub? Have you tried moving the sub around in the room, or even better, did you do the sub crawl?
Hang in there and lets see if we can help you get your sub/speakers dialed in.
