Russ69:
I was using the tuner inputs, but the Kenwood tech I spoke with had me try the other aux inputs. I think there are two additional ones, and it was the same volume regardless of which aux input I used. I definitely did not try the phono input because it requires three wires, one of which is a ground.
The Kenwood guy looked up the specs on my amp and told me what he thought the problem was. I could not repeat back to you what he told me, (amperage, ohms, etc.) but basically what he told me was that when my amp was made in the 1980s, tuner output was not as great and tuner perfomance was not nearly what it is today. The older amp was designed to work with and compensate for the tuners available back then. He looked up the specs for the modern Sony tuner I purchased, From what I gathered, it was either about 10x more efficient or powerful than the tuners the my old Kenwood amp was designed to work with. I hope that makes some sense.