I send them to the mains as my V2W's are boosted for extra boom in action movies. Also, I believe the powered subs in the 5 are of higher quality than the Earthquake. Finally, I believe that two sources for bass are better than one. Only one way to find out, is to try both modes yourself. You may also want to have two modes, one for pro-logic movies that sends bass from the center to the earthquake and one for music that sends the bass to the 5's.
Let me know about the phantoming thing too as i am really curious -- I hear that people have different results with different systems, and I don't personally know of any other 5 owners. I find phantom mode to sound much better.