to a pre-amp (for some reason)
Have we ruled out the possibility that this extra step he (for some reason) took could have affected the signals differently?
Could either be on purpose or by accident, but I think it’s likely to be the case.
Different pre-amp settings could give different results. If not on purpose, it could accidentally happen if a knob got turned a bit, or possibly even if a knob was electrically unstable, thereby drifting in value over time.
Other changes in internal hardware components of the pre-amp could possibly happen caused by changes in temperature or something else (?)
Could the slightly drifting volume value point towards a problem caused by the pre-amp?
disableable
Interesting word