Commenting on this point again, the "gain" control doesn't control gain, even when as in this case it's labeled as such. Audio amplifiers have fixed gain(the gain on a balanced input should be 6dB lower to compensate for the doubled voltage, i.e., 6dB more, on a balanced preamp and cable setup), typically about 25-30dB, which increases the voltage coming in from the preamp stage of the receiver a bit less than 30 times. What the control knob does is to change the portion of the incoming voltage which is let through to be amplified from nearly zero to nearly 100%. This is a variable resistor more correctly labeled a "level" or "volume" control.
There generally isn't a good reason not to set this control all the way up so that the maximum amount of the incoming voltage can be used. The amount of the incoming voltage, and the overall sound level being used, would then be controlled by the level control on the receiver. This would especially be the case where the amplifier level control apparently has a design problem at some lower setting.
