Measure the voltage coming into R1.

R1 according to the schematic would be the 1kohm resistor in the signal path. I don't think you mean to measure there? If you could mark it <3

 

Also in radio circuits, there often existed the practice of wiring a 0.1μf capacitor across the large electrolytic in order to de-couple them from high frequency.

 

Also in radio circuits, there often existed the practice of wiring a 0.1μf capacitor across the large electrolytic in order to de-couple them.

Interesting to hear that this technique has been used outside of hi-fi. Everything just gets called "snake oil" when it's about hi-fi. I also asked in the DIY audio forum about this and some people said that it's actually counterproductive. They did some measurements and stuff. If I understood correctly, their main argument was that the length of lead out of the bypass capacitor would add too much inductance to make it worse. It seems like BS to me.

 

Please measure the voltages at the legs of the two 5W 100Ω white ceramic resistors with respect to GND.