94astro - The DSX80 has a known problem with the COIU-A trunk card which affects longitudinal balance and creates the humm. You definitely need to ground ALL three of the ground lugs inside the DSX cabinet, but that may not solve your problem.
If you are a certified dealer, you can contact tech support and they will send you a COIU-B card which will correct the humm. Also, you may have to upgrade the system to .28 software.
Bill
