I am just wrapping up a bunch of work on my Space Station. I had some speaker hum. I read through a bunch of sites and found some suggested ground related feedback through the contacts in the backbox. I removed the screws on my sound board and the hum went way down. I ended up installing nylon washers on each side of the board. That helped a bunch with the hum. There is still some there, but it is very faint compared to what it was. Hopefully it helps someone else.