The crossover simply specifies where the sub takes over from the speakers. Below 40Hz for example, the 500 takes over from the M60 & VP150.

I personally would tweak but I would keep careful notes of what I hear. And I would trust my ears over measurements. Use the measurements as a baseline to start your tweaking.

I'm envious of your 3808 by the way. Maybe MultEQ could help out with some of my issues but I kind of doubt it.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated