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".
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".