Preamp issue?

My Cary Slp3 started to​ act up.
Few days ago I noticed something was off, but didn't have the time to listen critically..... yesterday it was even more pronounced but still not obvious...I realized that right channel is a tiny bit lower level than the left channel.
I eliminated all obvious issues, like bad connections, balance knob turned, etc...
I don't have spare tubes, and before I start opening the chassis and switching the tubes, just want to make sure this is even a possibility.

Thanks