Tube amp bias adjustment advice

You should also move the speakers left to right to see if the issue moves with the speakers. That'll eliminate the speakers as culprits. Simple enough.

1. at least it is a constant rather than a damnable intermittent problem.

2. when did this start? was it balanced before with a prior amp? With this amp before you changed the bias? this amp before you changed the tubes? did you move your speakers in/out of position? speaker wire connections checked?

3. anything else changed prior to the amp/tube/bias change? IOW, if nothing else was changed, then no need to suspect other equipment in the chain prior to amp and speakers.

4. but, even if not changed, a cable could have been inadvertently disrupted. my most recent imbalance turned out to be a cable, a very good cable, but became slightly loose on the rca input. Squeezed tight with pliers, problem gone, then replaced that pair with tight ones.

5. did you change which inputs you used on a preamp or integrated amp? gotta think that thru.

........................................

a. swap all left tubes with right tubes: problem still on left? not the tubes.
b. change speaker wires on amp, problem moves to the right, or still on the left? 
c. without changing anything else, move the speakers themselves. problem still on the left? good, speakers are fine!
d. put the speaker wires back correctly l/r on the amp.
e. narrowed down to ___ possibilities?
 

Room acoustics can be a bear to deal with, especially so when the room has to serve other family uses, or, for example, meet the decorating needs of a spouse. That said, small changes in speaker position and adjusting things like toe-in can often have a dramatic effect even if you can't add much in the way of wall treatments or the like. 

How about using your cell phone, buy a SPL plug in mic. Calibrate the mic and do a test or two. Quit guessing and see if one is actually louder than the other.

https://www.amazon.com/Dayton-Audio-iMM-6-Calibrated-Measurement/dp/B00ADR2B84/ref=sr_1_2?crid=3KHWP

ES posted this little mic and it works quite well. 19-20.00 usd

Order now and Prime usually gets it there in a day.
How are those ears? Make sure.

Most important, make sure you don’t have a speaker wired out of phase or something simple like that. A wire that is mismarked, or hooked up wrong.. Being that far off center is more than a balance issue.
Balance is time arrival, somehow that’s being messed with, right?
Phase shift, will cause a lower spl and late arrival..

Check for proper connection on the monitor section to the bass section.
Make sure they are in phase.
Check the SPL, make sure.. 20.00 one day...

Regards

My speakers are 20 inches and 23 inches from the back of the speaker to the back wall but if you measure from the front of the speakers to the sitting position they are both exactly 84 inches. Measuring from the back of the speaker to the back wall is a good starting point but if you want to be exact you must measure from the front of the speaker to the sitting position I’ve been doing this for over 50 years and I have yet to have the same measurements from the back of the wall to the back of the speaker there’s always some adjustments needed I hope this clarifies my position. Warmest regards