If it's done properly, supposedly epoxy potting will eliminate microphonics. Maybe Gibson didn't do a good job of it, if this is a common problem.
That is what I suspect- I have spoken to one other guy who had the same symptoms and were gone after wax potting.
Here is the crazy thing, though.
I took the neck pickup cover off yesterday to have a look. I noticed the cover had not been screwed down very tightly and I was actually getting a bit of mechanical vibration that was audible when played acoustically. When I put the cover back on I screwed it down tight. The acoustic vibration sound is gone now as far as I can hear. The pickup issues when amplified seemed less as well- it still will feedback at high volume and gain, but not as readily. The amplified background rattle is gone and the distorted echo seems to be reduced.
Is it possible that the mechanical vibration of the pickup is at least partially to blame for microphonics symptoms?
One thing unabated though is the annoying earth hum. My LP OSB does it too. Don't Gibson bother to earth their basses properly anymore? My Epi Vintage Pro TBird is silent beween notes and it cost nowhere near what the Gibson's did.
I have actually tracked down a genuine SG bass neck pickup for a reasonable price in the process. I think I will pick it up in case.