This sounds like RF interference getting in to the tuner. It could be coming in through the grounds, DC, or coax. Try the tuner with the amp off and see if it still has the issue; it will probably work fine. Also be sure to place the tuner in Semi mode; you don t want the tuner starting while the amp is on. You may be able to reduce RF interference by adding RF chokes on the DC cables going to the tuner and the radio.