The HPA-9's variable gain slider?
The slider... Does that effect only the headphone level? Or, will it also effect the output level to an amplifier?
If I want to use the HPA-9 in the the preamp mode? How should it be set?
I tried finding an online manual, but there was none I could find at this time.
I think you are looking at the HL and LL Headphone Switch. It is not a variable gain slider.
I think it only affects the Output to HeadPhones, but I don't have one here to test.
It is for High Level and Low Level Gain for the Headphones. It is a selector type switch.
I do have a copy of the manual and this is what it says:Gain Selection
High gain & low gain selection switch. Select the desired gain setting to fit your earphones and headphones spec.