One way to tell if the sub is interferring with dialog would be to turn it off and see if you can hear the center better.

Generally I have found that the surrounds and subwoofer channels are too loud in almost all consumer's systems I have heard, relative to what I generally hear on a mixing stage. This does not mean that you shouldn't adjust your surrounds to your liking however! The subwoofer especially is actually relatively mild sounding when a film is mixed. That doesn't stop me from cranking my subs (4 18" ones!) when I want to have fun at home!