You could do but you'd have a hard job balancing the fronts to the centre & surrounds unless you bypass your pre & use the volume control on a processor.
This is the big problem you see, keeping all the channels balanced The only way to do it properly is to use a volume control that does all 6 channels at the same time. You also need to balance everything so need gain controls which once set up never need to be touched again.
I fully understand what you are after now & it's a pita