I recently purchased a used ST-70 in mint condition. I've listened to the ST-70 quite a bit and I'm hearing a faint sizzling noise coming through the speakers. I purchased new tubes, tried different speakers, adjusted the bias voltage, also grounded the chassis and the sizzling is still present. I checked the quad capacitor with a fluke multi-meter, each of the four quadrants to ground read approximately 150K to 220K ohms. All measurements were taken with all of the tubes removed from the unit and of course no power. Any help would be greatly appreciated!