My guess is that since you have the ability to set the speaker sets at different levels, then you also need to the ability to specify what frequencies below a certain level should be sent. In a setup where you only have one option, it's clear, everything below that setting goes to the sub. In a MVC setup, which setting determines what goes to the sub? With the mains at 40 and the center and surrounds at 100 for example, does the AVR send everything below 40 to the sub, everything below 100 or both? By specifying the crossover for the sub, then you can dictate what is sent below that level. My limited education in this arena leads me to believe that if I have the mains set with a crossover lower than what I have for the sub, then I will get better "blending". Whether this is true or not, I have no idea. This could be entirely a placebo effect to keep us A type personalities happy. So far, I haven't been able to discern any major differences.
Experimenting sure is fun though!
