Enhancing voice or dialogue

I am quite happy with my low end system, but one aspect just doesn't satisfy. I have difficulty getting the voices loud enough to be clearly heard over the background music and sound effects. I have to turn down the volume on the subwoofer and turn up the overall volume on the receiver to get the voices to come out, but then the bass is a little thin. I'm using a Pioneer VSX-517 receiver that has a digital coax connection to a Toshiba SD6100 HD-DVD player. It runs a Polk Audio 5.1 speaker system with a powered subwoofer. I've selected dialogue enhancement on the receiver and it helps but it's still not satisfactory. How can I change the setup to get the voices to come out?

You increase the volume of the center speaker, where the voices are directed.

You increase the volume of the center speaker, where the voices are directed.

Thanks. I did that a while ago with the "dialogue enhancement" switch and altering the distance entries in the setup, but there is no "turn up volume on center speaker" function on this amp. What finally worked was changing the cross over frequency, I had it set too high. In addition I changed the speaker size entry to "small" I think. There really should be a relative volume adjustment for the center speaker, but it's accomplished only by messing with several menus, it seems.