You might consider that your hearing changes day to day, and throughout the day. It is likely not your equipment at all, but you. Humidity levels, exposure to other noise, barometric pressure all affect your hearing.
Dirty power could also be the culprit, although you would likely hear differences more frequently, or at different times of the day if that were the case.
Dirty power could also be the culprit, although you would likely hear differences more frequently, or at different times of the day if that were the case.