centering the human voice

Do you have the space for room treatments?

If not, it may be time to use the balance knob, if you have one.

If the room physically or acoustically is not symetric, you might end up setting speakers /pre settings differently to compensate and center voice..........your adcom pre has balance knob try it as Face suggests, tell us how did it go
Luis

i am now using the copland cta305 pre which does not have the balance control

Can we assume the voices you are trying to center are recorded that way?

You should be able to locate a voice in the soundstage with certain quality recordings but no guarantee voices are always mixed in so as to occur in the center.

Your room may well be a factor in an unbalanced soundstage left to right but I think that would probably affect more recording elements and not be isolated to voices only.