Power Amplifier Modification
Your variable resister is for setting up the idle current in the main powermosfet bank .More resistance means more gate source volts meaning more current .Mosfets are very spready so a fixed resister would not be good for a population of amplifiers,hence the variable resister .Once you have set the idle current you could use a fixed resister of the same value as the variable one .This fixed value would be good for your amp with your fets but probably not for somebody else .Fixed resisters are more reliable than pots .If you had to replace the expensive fets your resister value would probably change .