I have to give more precisions. When I play at room, with the drive channel, I set the amp general volume at maximum 2, saturation on 2-3 (first quarter), boost activated with the strat, bridge pickup and low impedance amp input, I have a pretty sound, when I switch on neck pickup, the sound is something too muddy (sludge) and I lost the definition. No issues on guitar electronic.
I will prepare some pictures to explain more in details.
The clean is not concerned.
The manual explain that we don’t need to change the bias when we change el84 with another supplier. In my case, the quartet was changed with sovtek last year.
I see what you mean. I’m struggling to suggest something useful in this situation. The amp and is amplifying the input signal it receives, if the only thing that is changing is the pickup selector switch then you have optimised your tone for a single pickup. It is unusual to have such a poor tone from one pickup and enjoy the tone from another. I would consider changing that pickup!
It’s really nothing wrong here, it’s just you need to use different EQ settings on the amp so that both pickup settings provide a good tone.