<< but it's not the RX audio which is the problem - it is my over-excited shouting down the microphone when I hear a rare DX hi hi! >>
With the headset mike an inch from your mouth, you won't need to shout any more.
I think you are going to have problems with a mike run that long unless you got to balanced lines for the mike run.
Good luck with the project.
FWIW, a couple of very rude operators cause me to miss a JT and a VY this evening. @#$%^&%