Prima Luna Dialogue One Balance problem

My PL Dialogue One Integrated amp has a balance problem.

Left channel is less lively, sounds probably a couple decibels lower and highs are not as much present as on the right channel.

I have swapped tubes, tried new sets of spare tubes (driver and power tubes), swapped spk and ic cables and even speakers, tried different sources. I have PL EL34 and Genalec KT77 output quads and RCA NOS 5751 + RCA NOS 12AU7, as well as EH 12AX7 + EH 12AU7 driver pairs. The problem remains.

Any thoughts?

I see that there is a single trimmer potentiometer on the adaptive autobias circuit board. Should I try to adjust it as on the link here? http://www.primaluna-usa.com/autobias-adjustment/

Does autobias alignment have an effect on imbalance?