Ok to adjust balance to achieve center vocal image?

Yes indeed. Trouble shooting could begin on either end of your system.

On the front end, see what happens when you swap right & left phono inputs. You may or may not find the answer there . . .

As Tim described, you can do switching starting at either the front end and working down the line, or the other way (from speaker back to source).  This will allow you to isolate the source of the problem (assuming it is in the component stream and not the room or your own ears).  This switching also involves moving tubes from one side to the other to determine if the weakness is in one pair of tubes.

Right now, I have a substantial imbalance that I've isolated to my linestage, and it is not the tubes.  Because I don't have any other obvious problem with the sound, I have been utilizing the balance control to compensate.  At some point I will bother to have this looked into by someone more competent than myself.

I think balance control is an essential feature.  There is always some some imbalance, particularly with phono cartridges.  It is amazing how small an imbalance is actually audible.  When I had a Levinson No. 32 linestage in my system, I could change volume and balance by as small an increment as .1 db.  While a whole db of volume change when playing music is not reliably audible, a change of as little as .2 db in channel balance IS audible.  That is why I had the builder of my tube linestage include balance control (I also insisted on remote volume control, something he was not normally inclined to include in his designs).

     Yes, thank you toothroyd.  A faulty interconnect cable could also be a possible root cause just as improper stylus tracking in the groove could be a root cause. The process of elimination should enable you to narrow down the possible root causes. 

Tim

Lots of ideas here.  If it were me, easily my first theory — especially since you said it was not previously doing this, presumably in the same room and with the same/similar gear — would be that my azimuth had fallen into a leaning position and the stylus was favoring one channel by a smidge.  Lots of ways to check and adjust that, depending on your gear.  Worth a few minutes on googling “turntable ... azimuth.”  My second theory would be what someone above said: my room is such that I need to bring the weak side speaker an inch or two closer to the listening spot.  This can be such an easy fix that it feels like cheating.  

Wayward imagery can be caused many things that may drive mad someone. I would include the following for checking,
Use a cd player to confirm that image is center.
All Recordings are not the same, check with a test record for center voiceNot matched tubes in phono stage, swap tubes L/R to check that image is the same or not.
Align correctly your cartridge, overhung, hta, antiskating (use low), azimuth.....
In most of cases your answer is there.
Speaker positioning, toe in-out, same tweeter axis, moving front to back as rooms are not perfect.Swap L/R cabling.

G