2 easy steps to make this horrible tragedy go away as easily as possible.
1. Buy epoxy
2. Glue cap back onto subwoofer
The end...
I fixed my PB15, which had the cap fall off, and the cone rotted out. It works fine. I bought the PB15 through Stryke, and when the time came to get help with service, John was gracious enough to tell me that it's not his problem since he no longer sells Lambda Acoustics products. Asshat, plain and simple. If he's willing to help you glue the cap back on, yay, I'd still say do it yourself.