pulseaudio – How do I set default +3dB boost to my rear speaker in a 5.1 sound setup?

So I use 5.1 surround sound system, plugged in the back of my motherboard (3 Jacks), as well as headphones plugged in the front of the PC for gaming with the boys.

The thing is, every time I switch back from Headphones/Stereo to Line Out/5.1, the custom levels I had set in pavucontrol for the rear speakers are reset.
Since my rear speakers are quieter than the front ones, I need to apply a +3dB gain for them to be at the same level as the front ones.

Therefore here is my question : how can I change the default behavior of PulseAudio so that, when Line Out/5.1 output is selected, a +3dB boost is applied to the rear speakers ?

Thank you.