Dave - If I connect only the GND and B- to the 500-C chassis, and then I touch the LEFT IN and RIGHT IN from the Unity Gain board to the center tabs of the volume control without disconnecting the existing leads to the center tabs, I get a reduced volume but no distortion. If I connect the LEFT OUT and RIGHT OUT from the board to the center tabs, I get the reduced volume AND THE DISTORTION.
I also found out that if I touch either of the IN leads from the board to either center volume tab (again without removing the existing leads on the center tabs), and I touch with my hand either of the OUT leads from the board, I get the distortion. It sounds like a 60 Hz ground hum.
Did you want me to do these tests with the existing leads removed from the center tabs? Thorne
UPDATE: Some other interesting facts... I traced the B- through the Unity Gain board. B- coming into the board reads -22 VDC. That goes into pin 4 of the LF353N chip. Following along, on the other side of the 47K resistor, the reading is -10.5VDC. That voltage goes into the 1M resistor, which goes into pin 3 of the chip, at which point reads -1.5VDC. Following the -10.5VDC, that goes into another 1M that goes into pin 5 and at pin 5 reads -1.5VDC. What's interesting is that pin 7 sometimes reads 0.6VDC and at other times it reads 0V. But when it reads 0.6VDC, the other side of the .22uF 50V cap always reads 0VDC. Does that make sense? The same test point reads between 2.7mVAC and 3.1mVAC. Same readings on the other channel.