Speaker position question?

Hi all,
I have recently moved my system and speakers, mostly for the better.
One problem i am having which is driving me nuts is the center image is slightly focused to the left,my test tracks with a solo vocal and or instrument cleary shows this.
I guess this could be a result of having one side wall(left) just plaster board and wood and the other wall(right) a curtain.

As my speakers weigh 140kg and are on spikes it's tricky to just play with the toe-in. I'm sure I've read before by toeing either the left or right speaker in or out you can shift the center image.
Can any of you guys let me know what is the best method for shifting the image?

Also i have the speaker toe-in so the axis of the tweeters cross just infront of my listening seat, i am thinking this is to much, i noticed when i move forward the speakers disappear better, and moving further away the music seem to appear more from the speakers.
Does this sudgest toe-in is to much?

Againg thank's so much for any advice.

Regards Rob.
Yes, symmetry is important. You might try toe-in that focusses the on-axis just behind the listening position, rather than in front. But, this won't fix your center image problem. Try setting the balance at the preamp—a mere quarter or half dB could do it. I suggest using pink noise and an SPL meter to get left & right speakers in balance—far more accurate than by ear. I have one system in an asymmetrical room. It requires the left channel to be .5dB lower to balance.
Beats the S*** out of me- but I will assume the listener is not;
1. Further away than the point of the golden eqilateral triangle and 2. he hasn't tried putting up a curtain identical to the one there.
It makes sense that the wall will cause that side to be more lively. A flat non absorbing/dampening wall typically reflects sound, compared this to a curtian.
My phono preamps don't have a balance knobs. Were you suggesting a resistor?
Sounds like you need to adjust the speakers a bit more. Like above, move the toe-in to meet just behind the sitting position. I just finished moving mine around, as I had the same problem. A fraction of an inch will make a difference. The distance from the rear wall was off by a half inch on the left side, and I made sure the speakers were level, which they weren't. Had to remove the spikes in order to play around with different settings, because each speaker weighs well over 200 pounds, but in the end it was worth it. The sound is centered as it should be now.
Post removed 
Thank's for the advice,

I did try hanging a blanket on the left side wall and the image is now more central.

I will experiment with toe-in.

Regards Rob.