Question about my Marantz 1150

There is a knob on this integrated amp that is labeled "tone" and it's switchable to an "in" position and to "out" position. I can't tell any difference in the sound between the two. Does anyone know what this control does or how it's used?

If you have tone controls (bass treble etc) I would imagine that the knob defeats them on one or the other position.

OUT switches the tone controls out of circuit.