Yes, they are designed to eliminate hum caused by the ground plug without removing the benefits of that 3rd plug. But $65 compared to 50 cents makes you think about how many you will need.
I had the same issue when I added my Behringer amp to power my subs. I went as far as making sure all 3 of my 20 amp outlets in the room were on the same leg of the breaker box. Still no help. I went to Wal-Mart and bought a 3 prong to 2 prong adapter for 50 cents. Worked like a charm. No...