George,

As we said, the hiss noise is independent of the Master Volume control button; this is, with the master volume at zero (full anticlockwise) the hiss noise has the same level. So the mic amplifier noise, at that position of the master volume (zero), is reduced to zero because the signal lines R and L are connected to the ground by the Master volume.
The noise, I guess, come from the pre amplifier after the Master volume control (the two OP and the internal fixed equalizer - I'm not refering to the equalizer we have access through the display; this is made by the digital circuit and software before the Master volume control).

Carlos Rodrigues