“Center” of soundstage is left of center…

OP:

Yes. If the problem persists after swapping inputs, the next thing to do is swap amp outputs. If the problem persists then it’s either the speakers, the room or your hearing. :)

I have this exact problem and tried many times to locate the source. After a few years I found out (hearing test) that I have a significant hearing loss in my right ear. It’s a fairly narrow bandwidth, but I barely hear anything in that band. It’s the root of my tinnitus. Music that’s engineered with instruments panned wide left/right is fine, but anything subtle has a distinct left of center image. 

Lots of good comments. Could also be one speaker is slight forward of the other

Did this just happen one day when the previous day it was centered?

I think I’ve found the answer. I screwed up how I adjusted the tube bias Dow the mono locks. I had it totally wrong. Now all is well. Thanks everyone for humoring me.