Amp Kenwood KA-990EX has a little disbalance between channels: L vs. R

[bibu16v]

As per title described.
Idle is now 9mV/channel as per SM specs, it was 14mV/channel, but I've tuned it down. Same issue was before tuning, no difference.
Changed speakers between each other, no difference, L channel is a bit weaker. Changed different audio sources, changed L and R channels (RCA connectors) between them, no change.
There's really no big difference, but still audible, I can correct it with Balance button going to 2/L ....it seems to diminish to dissappearence when increasing volume...I suspect a volume pot with some issues...said pot works very easy, with no parasitic noises...
Any suggestions/ideas will be much appreciated. Thx.

[Kowalski]

Is one channel also quieter on headphones?

[bibu16v]

Yes, it is, same thing using headphones...

What does this mean?