Need help on Aesthetix Callisto sgn.

My volume balance changes as time go by.
First half hour listening both on balance, after that my left channel is stronger, I have to reduce by 2 click on the left channel, then half hour later another 2 click reduce on the left channel. Do anyone have this experience,
I have interchange all tubes from right to left each at a time. Still same problem.
I guess I better call Glenn, but if anyone have this experience in your system, please help me.