3M makes some stuff, you can also get car audio padding for that purpose, like Dynamax (get Xtreme).
Try adhesive-back polymeric damping sheets. Used for vibration control, on pipes, cars (under the hood, for example).
Available probably at local (large) h/ware store. I got mine from McMaster-Carr.
Cheap and VERY effective in my system! Cut to size and use on front-plate, top-plate, sides, caps... (BUT BE CAREFUL NOT TO TOUCH EXPOSED PARTS OF CAPS WHEN YOU OPEN THE MACHINE).