Steve, I've experienced this as well and according to Ian it's something that needs to be tweaked with the timing algorithm in the DSP. I believe what you are hearing is actually the driver "bottoming out" not conflicting with the wires inside or any other obstruction.
You only notice it at very high volumes and on scenes with extreme bass ("The Pulse" server room scene will bring it out quite clearly). It's quite rare to hear this because it requires a combination of very high volume and very deep bass, but it sounds like they're working this in future code revisions as well. Even if they don't it's not that big of a deal to me, but it is on the Axiom radar from what I understand.
JC told me that the amplifier in my replacement is updated to fix this issue. The thump is still there when switching off, and he said that I'll still get another amplifier once they've resolved that to their satisfaction.
