[BeoTool]

I’ve had similar hum issues on BL 8000s when using the RCA jack, but not when using PL.  In my case the RCA jack began to loosen on the PCB board.  At first the movement is barely perceptible, and the hum is soft.  Over time as you move the speaker or touch the RCA cable, the jack wiggles more and the hum becomes an uncomfortable yowl.  A mechanical issue surely caused by endless plugging/unplugging RCA over 30 years.

 

I re-soldered the feet of the RCA jack to back to the board, and (fingers crossed) all is well.