Audison Voce 5.1k disorted Signal

Hello, i have a Audison, on the Channels A ( A-Class) i have i disorted upper Sinewave, on the right channel its the lower sine wave. I expect a problem in the preamp board. It doesnt change, if i move any switch or HP switch. When the amp heats up, it becomes more. Maybe somebody had this failure allready. Also the Sub Channel is damaged, but i removed the Drive board for that channel, and the amp runs fine for a year. Maybe also somebody know something.
 

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