IMAGE PROBLEMS VOCALS OFF CENTER

If you are convinced the room is acoustically symetrical and your physical arrangement is good, and you've switched all cables and equipment including speakers back & forth with the same off-center effect, I think you're back to the room. There is something you're missing. (There is an old saying that when you hear hoof beats you look for horses before you look for zebras - unless you're in Africa.)

Would it be a dumb question to ask if you've taken an album where this effect is pronounced and listened critically to other systems outside your home? If you hear an off-center image there, it could be that you're experiencing a change in your hearing ability due to age, past hearing damage or another medical issue. Been to an audiologist lately? (I do recall you saying others have heard this, but never hurts to check with your doc.)

If it is external EFI that is causing the problem, it would presumably be picked up by the speaker cables since they are the only ones not physically together from source to amp (unless you have outputs to monoblocks by the speakers.) You could try doubling the length of the speaker cable and looping a section over to the other side and see if that causes a similar shift in image. (You might have to use cheap wire for this test to get the extra length.)

You could also check out alternate speaker cables. Some offer better shielding, akin to a coax or balanced cable type shield.

Or, you could move the equipment to the opposite end of the room and see what that does to your image. If that does change things, you could be back to re-examining your acoustics or possibly have revealed an EFI source that is stronger at the other end.

Have you spoken to Obsession Audio about the problem
What do they think it is

This sounds like a classic phase problem. Yes, try a mono recording if you have one, or a regular stereo one if not. Switch the positive and the negative on one (only) of your speakers. Perhaps you have purchased a (used?) pre-amp/you don't have the manual/ and it reverses polarity. Many do. In any case if it sounds better, with centered image and better bass, that's it.
If that doesn't get it and you've tried the above, go ahead and make speaker placement adjustments, move one closer/forward, angle one more than the other (this makes a big difference in centering), because your room is not perfect. Things don't have to be perfectly symetrical, you neurotic engineer.
(I know, we're all neurotic.)

If you have tried swapping and eliminating all hardware (checked voltage output on standard tone out of each amp etc) then clearly it can not be the equipment. It is you or the room. If you know for sure others hear it (or you have tested your hearing with same db output from each speaker and you dont hear the difference) than it is the room. If it is the room, try: (i) measuring dbs with same db tone CDs, right and left channel using an audio shack db meter and if that does not make the issue clear (ii) download the room EQ wizard (buy a mike) and measure not only immediate dbs but also map out various milisecond scenarios. That will allow you to see if reverbs are causing sustained dbs beyond the immediate timing (often the problem.) If that is the problem, get your money back from your room designers and start experimenting with room treatment (forget equalizers for reverb problems, much less helpful if at all.) If that is not the problem, then move houses!

By the way I assume your issue is not only vocals but pretty much all sounds. Otherwise, it is really mysterious.

Mihalis what is the reference position for all sounds,and where are they supposed to be seen on the landscape of audio recordings? Even with vocals, which you would assume to be placed at perfect center there may be a blur in the acoustic bubble which may place it off to the side. Where does that blur originate, is it a singular point of the chain or is the cause of many? From the recording space, or mics or mix, to the press to the consumer electronics to the playback room, all are probably to blame. I have been fighting these issues of image solidity for years. The more powerful microscope you have the more crap you can see. So is the same for high resolution audio, darn it.. Tom