Had this problem for years with my much loved SS5. Only just now i decided to pull it out and fiddle with it some more and have the "humming problem" SOLVED
. See description and PICTURES below
-remove the 2 small philips head screws from the speaker plug.
-first note which wires go where, i took a picture.
-take a small object to "lever" up the tiny plastic "fingers" that hold the plug wires in the plug housing, i used a small file, but you can try fine nosed tweezers.
-whilst doing this pull back at the wire at the same time to remove the metal terminal from plastic housing.
-repeat for the remaining wires.
i used small electrical snips to "pinch" the tiny metal plugs to make the connection tighter. I pinched it till there was almost no space between the contacts then re-insert wires till it clicks and it is then locked in place.
replace the housing with the 2 screws, and when you go to plug the speaker back in you will feel it is "tighter". No more humming at all!
-repeat for other speaker. The process of "pinching" may take some practice to get it to fit nice and snug around the connecting pins in the SS5 main unit.
Note; pinch from the bottom flat surface of the metal, and you will see the contact coming closer together. if you do it from the top, it will simply spring back open when you release the pressure.
hope this helps, this problem had me scratching my head for a long time, i even tried re-wiring the plug, when it was simply the metal pins in the housing!