Manual gain controls on Hafler 9180

Yes folks, you read it right here! I have a Hafler 9180. "Still running after all these years..." (since 1994)
Can afford to replace but the sound pleases me so it remains. 

Enough background and introduction: the point of this post> 

I know nothing about audio electronics, but am willing to learn...)
The Hafler has a manual gain control for each channel located on the front panel. Is there an "optimal" setting for these? What sound differences will I hear by adjusting these one way or another?

Thanks to all learned folks responding to my query.!   

These controls are input signal attenuators, which can be used to better match the preamp to the power amp. They can be useful if your preamp has more gain than needed. This can be a problem when using higher output sources such as CD players.

A symptom of this sort of problem is an overly sensitive volume control which doesn't permit a fine enough adjustment of volume. This can render a remote volume control virtually useless.

My guess is that overall sound quality is probably better when these controls are turned all the way up because turning them down introduces a resistance into the signal path, but I'll leave that issue to more "learned folks".

I would try to position the controls on the amp such that I advance the volume control on the preamp to about the 2 o'clock position for the loudest desired system volume.  Hopefully this can be done such that the attenuation on the amp is minimal.