I agree that if you can distinctly hear the rear speakers, they are too loud.

Having the fronts and rears together on the same amount of delay is very nice, obviously better than if it did the fronts only. However (and I know we are hardware limited), delaying the rears seperately compared to the fronts is also nice. Where the left / right delay sits you in front of the stage, as opposed to the left or right side of it, changing the front / rear delay brings you towards and away from the stage. It can make it feel like it is shrinking way out in front of you, or that you are almost right on the stage itself.

This could be done with a variety of aftermarket components, for relatively cheap you could wire in a guitar pedal, if you didn't mind the labor involved actually installing it and converting it for 12V use.