Is your receiver set this way for Dolby Digital, or is it set this way for a THX dsp? It sounds like you've set it up for THX, but left the Dolby Digital to languish with default settings. Could be wrong here... Also, you should cross the sub in the receiver programming at the same level as the speakers; ie 80 Hz. The sub's crossover should be disabled.

I am the Doctor, and THIS... is my SPOON!