It's known that the tone knob on the JRC NRD-545 has an effect on both the high and low audio frequencies. What isn't as well known is that the control determines how much audio will be passed on either side of 800 Hz. Since this is the radio's default CW offset, this characteristic gives you a most useful variable audio filter.
This spectrogram shows the tone knob being turned slowly to the left. Note how the action is gradual at first, then becomes a sharp cutoff.
This isn't documented in the manual, and few people know about it. I found out by doing the exercise you see here. Now, you know it too. Your NRD-545 has a Secret Audio Filter.
Image made with Spectrum Lab.