centering the human voice

You may also try listening to a radio station like sports- talk or news or a talk show, or commercial. These are generally mixed in stereo and sound will most likely be centered. If you have music you are very familiar with as far as sound placement you could try that. I have test cd's that I use when breaking in gear and for demoing and they are tunes I know intimately. Hope these suggestions help.

"speakers do not have to be placed symmetrically in a room to sound right"

No doubt.

Good thought Reubent.

And try changing +/- on the speakers only. Keep right amp input to right speaker etc.

First, and I apologize for stating the obvious, but your listening position must be centered between the speakers and to start with, the speakers should be identically positioned in relation to the rear wall (and if possible, the side walls). You don’t state your room size; if you have a relatively small room I recommend setting your speakers/seating in an equal triangle (tweeters to “ears”). You may want to bring the speakers apart from one another (increase the six foot space) – also, if they are towed in, try facing them straight into the room; if they are not towed in, try towing them in slightly so that the tweeters fire just to the sides of your ears. You’ve probably already done all the above, so have you tried making small adjustments to the left speaker only (moving it back in very slight increments)?

Something else to consider is whether or not each of your speaker cables are the identical length. I agree with the above regarding correct phase. It’s possible that you are getting some first reflections on the left wall, while the right wall may be diffused or absorbed (i.e.: furniture placement). Again, try adjusting the left speaker (and as stated above, treat the room).

You might post a photo of your setup. IME if one speaker has corner loading (a sidewall and frt wall) and the other side does not (only a frt wall) you will never achieve a consistent center balance.
A controlled directivity monitor versus your full range wide dispersion speaker would be a better match to your surroundings.