Accuphase C-200 "COMP" button - What does it do?

I'll take a stab.
Maybe "Compensate"? Similar to a "loudness" control?
This would boost both bass and treble and is designed
to allow one to listen at lower levels (like late at night)
and still get some extention at the freq. extremes.

That's exactly what I was thinking. I would be curious, literally just for curiousity's sake, what the boost is, frquency-wise, you know.

Thanks.

for use at lower volumes

To add to the above:

Hearing acuity is subject to (if I remember correctly) the Fletcher-Munson Law. This is a measure of the reduction in hearing sensitivity to the high and low frequency extremes as the volume goes down.

According to anthropologists, the reason for this has to do with survival. i.e., predators produce mostly mid-range (crunching leaves, heavy breathing, etc.) and so by accentuating your sensitivity to the midrange at low levels (like at night for instance) you have a better chance of getting away before you get eaten.

For a similar reason, our visual sensitivity shifts to blue in low light at night (the color of moonlight.)

In the old days, those loudness circuits were often called loudness compensation.