IMAGE PROBLEMS VOCALS OFF CENTER

I can sympathize with you on this one, in my case totally room related. It could be the volume pot on your preamp (or amp, cd player, etc.) mistracking, it seems you've checked everything else. Otherwise it sounds like room or speaker postioning.

A preamp or linestage with a balance control is the ticket. I go through the same thing with tube variables, recordings, and all the other things can can screw your day. The equipment designers that leave off balance controls in the interest of purity are mad.

Another method you might try is called Masters. Sumiko has some dealers who are trained for speaker placement that matches their placement in the room according to equal pressurization. Rod Tompson at Soundings in Denver is rather adept at this.

Shadorne wrote

"If you have swapped interconnects (between source and power amp) and you have (in a separate test) swapped speakers with their respective cables (left channel to right side and right channel to left side) then you have eliminated your system as the source of the problem."

So did you ever swap the speaker connections so that the left output of the amp is driving the right speaker and the right output of the amp is driving the left speaker ?
I didn't see an answer to this question, and in my opinion it is the key question.

Try what Seandtaylor recommends above. Then, also try forward toeing-in /very slightly/ your right speaker -- i.e. the speaker the vocals are closest to.

Swap the speakers. If it remains, it is not your speakers.

Completely swap the speaker cables:Disconnect completely and put the right cable on the left side (amp to speaker) and the left cable on the right side (amp to speaker). If it remains, it is not your speaker cables.

Now swap the speaker terminals at the amp. (use the right amp channel to drive the left speaker, and the left amp channel to drive the right speaker). What was being presented to the right side of your amp is now being presented to the left side, and vice versa.
If the sound DOES change (you hear a shift past correct to the right side) Your Amp is OK.

IF the sound does not change, (same left-prevalent image) Your amp, somehow, it to blame. Check Bias on the tubes. Swap JUST the power tubes.Listen. Then swap just the pre/driver/input tubes. Listen. IF the Amp is auto biased, the circuit could be pushing a power tube too hard because of a weak driver/input (Maybe, someone else chime in here).

Completely swap the right and left ICs between source and Amp. IF still there, not your ICS. Now swap the ICs at the rear of the source. IF it is still there, hire an exorcist.

By the way, I had a similar problem a while back. Hearing tested. Everything done. The only difference between my situation and yours is that no-one else heard my issue.

I would suggest that you get as many different opinions (without leading them to hear what you hear) by asking "is teh image center, right, or left"

My problem is that, while my hearing is fine, I am right eye-dominant, and tend to turn my head to the left while searching Visually for the center image in the soundstage.
Perhaps not coincidentally, my right ear is slightly farther from the tip of my nose (or the center of my face) than my left ear. SO, I have to turn my head slightly to the left to present my ears at the same plane. When I do this, I hear the image in the center, but my head is oriented slightly to the left. If I remain focused on the center point by gazing slightly right (so my head is turned slightly left, but my eyes are straight ahead) I hear ( and "See" which is just as important) the image as it should be.

Don't measure your head if you aren't prepared to see a couple millimeters of diference!!!