Roland, at least in the past, always used a floating ground system, which would preclude a ground fault conflict.

Graham, if I were faced with this, the very first thing I would do is unplug everything, everything from every connection. The only thing that would be plugged into the surge protector would be the Bose. Then turn the most on, turn the volume to the 12 O'clock position and connect your RCA wires to the Bose and nothing else. Now, I looked at those wires you are using and they appeared to be unshielded, but there is no way I can tell by looking at the vids.

Next, plug the keyboard into the power strip, turn on the keyboard, and if there is no hum, turn the volume all the way down on the Bose, connect the keyboard using those RCA cables, then turn the volume of the Bose back up to the 12-O'clock position and see if there is a hum. If there is, turn everything off, then try using fully shielded individual RCA cables. The cables you are using appear to be adapter type, but again, I can't tell from the vids. The hum almost sounds as if there is an open connection of sorts, but I would have to check them out using a multi-meter.

Keep in mind that even when other devices that are plugged into the surge strip are turned off, they are still connected through the ground connection of the strip. And, some surge protectors produce their own problems with the type of protective circuitry. You can also try using just a power strip without a surge protector to see if the same problem persists.

This is all just guesswork on my part, Graham, and I hope you can get this ironed out. Or you can slip down to Marathon Key after the first of the year, bring your gear and join in on a jam session at the marina, which will give me a first hand look at the connections and possibly solve the problem. Even if I cannot, the Keys are always a fun place to visit during the dead of winter.

All the best,

Gary