Thank you for the advice.
I'm pretty sure I understand the theory, and after getting a USB inspection camera in there, nothing appears to have come unglued, the bridge seems solidly affixed. I think its a combination of some of the things mentioned above, (top isn't stiff enough, braces are not stiff enough & too heavy a gauge of string). The truss rod was maxed out and there was an obvious belly behind the bridge, it was low at the front (rotating forward under string tension)
While it sounds quite pleasant, it's not an expensive or rare instrument, and I can't get a bridge doctor locally (will have to get one shipped from the US). Also the bass is not mine, I'm just trying to help out as a favour.
Since the bridge doctor will need a 9/64" hole anyway, I got approval, drilled, and made something up out of what I had laying around, it seems to be a marked improvement. I'll order a bridge doctor, and maybe creep another 1/8th of a turn in 24 hours and see what happens.
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