Thank you for responding so politely and comprehensively to my post, Terence. I was going to ask if the reason you hadn't posted anything new at that point was because nothing had changed in the interim, but I wanted to believe you'd made at least a little progress by now. Bummer, man.
Listening to the clip, it immediately reminded me of the effect old / charge-deficient batteries have had with the various EMGs I've used over the years. The same sound is evident in bass guitars with actives which're being powered by charge-deficient batteries.
I realise the chances of this being the issue for you are practically-zero, but I thought I'd mention it 'cause the similarity's uncanny.