IMAGE PROBLEMS VOCALS OFF CENTER

Take a hard look at the placement of furniture, rugs, artwork (particularly with glass in the frame), the balance of audio absorbent coverings such as cloth uphostery vs leather or plastic, and so on.

You might also try slightly different toe-in angles to help correct - one angle for one speaker and a slightly different angle for the other. The degree difference does not have to be much to have a big impact on image placement.

There could also be a defect in a cross-over network or driver in one of the speakers, or an off-value or deteriorating component in the source, preamp or amp. For example, on turntables, I've seen the stylus cantilever get knocked slightly out of alignment and that'll affect the volume between channels.

This won't help with the phono cartridge, but if you have a steady-state mono source (e.g., frequency generator) and a reasonably sensisitve multimeter, you can check the voltage being delivered to each speaker terminal. That might help narrow your search.

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.