If you still suspect the thermistors then just try snipping them out and seeing if it changes the fault. They do seem a very strange and unpredictable component to add to a volume control circuit and their only function (that I could think of) would be to reduce volume if the amp is running to hot.
An oscilloscope would confirm whether the signal level was varying across the volume control with time/temperature.
An oscilloscope would confirm whether the signal level was varying across the volume control with time/temperature.