I am not using my Flesher TU-170 right now, so I was able to put it on the bench.

From the schematic it looks like the input impedance should be about 2500 ohms .

I used tones coming from a USB sound card on one of my streaming computers for testing .

It starts keying the loop at about .1 volt of tones. That is about 6 on my meter.

It starts limiting at about 7 on my meter. That was such a slight difference in input voltage that I could not see the change on the 'scope.

My guess is  that it would work fine on  a stereo system 'line level' or any 600 ohm line level..

The tones from my tones from different computers run from .2 volts to .8 volts to 4 TU's that feed separate loops to monitor my streams. I use those loops to test Teletypes.

By the way I have had trouble using the Flesher on one of my big loops. When I would plug in a 28, the current meter would pin. Some day I may try to figure out why. Right now it was easier to go back to a Heathkit  HD-3030. That is why I am not using the Flesher.

Tom N3CRK