Well the 600 does sound good with music, its just that the 400 sounds better IMO. Jason's plan is a good one. Position a 400 between your speakers and listen to it with music. For HT power up the big guy and the sony. Its going to take time to optimize those three subs but with patience it should do wonders for your LF room acoustics. I like to use 3 different model subs in my larger HT and they work very well once you find the right placement and get them level matched and working in phase.

John