Curtis mathes stereo question

[Olorin67]

I dont work on solid state gear, but that sounds like a capacitor somewhere that has gone leaky, upsetting bias voltages.

[zeno]

Since it crackles loudly its father back than the vol control.
Probably open coupling caps, the same one in both channels.
Probably electrolytics. How about a few chassis photos & model
number ? Maybe we can help.

73 Zeno

[parvo]

You might want to try a little DeOxit D-5 on the selector and the pots. Capacitors may also be a problem at the age your Curtis-mathis set is.
A bad or corroded selector will put a damper on the sound too. Worth a try.