While doing some troubleshooting on an old receiver, I noticed that the MUTE R or L, when setting GEN 1, did not mute the selected signal when running running in the IDLE mode. I am not sure if this is by design or not, but thought I would mention it. Of course one can pull the cable from the appropriate input as it could cause a little confusion as it did to me for a minute or two (Why the heck is the 1khz signal showing up here ?) 
Hi @Var, noted and thanks for the bug report!
I was not sure if it was a bug or not, but am always glad to help with ideas for impossible improvements.