While it's always advisable to calibrate the relative levels of all your speakers, don't get too obsessive about the center channel. With some movies, dialog is mixed at too low a level because in the mixing sessions, the director and the audio engineer know it by heart (they've heard it so many times) and mix it too low. In cases like that, you will need to turn up the center-channel volume until it's clear and intelligible.
That setting may change depending on what movie or video you watch, so find the easiest way to adjust the center-channel volume level. Every receiver will let you do that, but you may have to go to the on-screen menu to adjust it. Usually an increase of 2 to 3 dB is enough to improve dialog clarity.
Also keep in mind that the sound quality of the center-channel speaker is very dependent on placement. All the tips here are useful, but try experimenting with the placement of the speaker. If you can put it beneath your TV screen, you may find the dialog clearer. Every room and setup is different and what works well in one room may not in another.
Don't set your center channel speaker to "Large." Use an 80-Hz crossover freqyency in your receiver's bass management setup. " may inhibit dialog clarity, and your subwoofer will carry all the important bass frequencies anyway.
Axiom Resident Expert