Picked-up a used Crown K1 and K2 from the same seller. They both hum while signal is going through them and goes away within a few seconds after signal stops (due to the Sleep Mode circuit). Also have a Yamaha P2075 from the same seller and it's quiet as can be the entire time under the exact same conditions/tests as the K1 and K2. Tried three different power circuits, lifted grounds, etc., and still the same results every time. It's weird both the Crowns have exactly the same problem, unless it's some sort of power supply problem and both amps were subjected to the same "A/C punishment". Any thoughts? Thank you.