centering the human voice

"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.

Try placing a mirror on the wall at the same hight as the tweeters. Sit in your listening position and have a friend move the mirror along the wall until you can see the tweeter from your seat. Mark the spot and do the other wall. These are the first order reflections. Place something at that spot to absorb the reflections-like a tapistry or some other acoustic treatment. This will go along way to cleaning up reflections and will improve presence, soundstage and frequency tone. For even better results, continue to move the mirror along the wall until you see the other tweeter-this is the second relection. Treat it the same way. Good luck.