Thanks a lot, Baron030, for your valuable comment and advice. And you're right, Wagner.
I'm just a great fan of vintage JBL gear, with no technical expertise whatsoever.
The whole purpose of my thread on this was to find some clue to this peculiar thump noise from speakers when JBL SE408S was powered up. And I think I know the so-called T-Circuit has something to do with the noise, thanks to Baron030's kind explanation.
I take your advice seriously, so rest assured. I'll take it to the vintage tech in my area if I see any problem with this unit.