Sometime I increase mine a little... not to the point where they seem too loud or out of place, but just enough to make them a little more enveloping.
I always end up setting them back to the 75 db calibrated settign though, because that is the ideal volume for much of the material I watch/play. However, If I feel like the surround levels are low or lacking on a movie I don't hesitate to take them from +1 db to +4 db on my receiver.
Again, I do the same with my sub.
I never turn the sub or surround below the calibrated level though...
I guess the center button on the 3808 remote just makes it so easy to adjust channel levels o the fly.
