Question about adjusting the balance

So what you data is telling you, as you described it above.  Is that you have a probem with either one of your highs or one of your mids.  My first thought was that it would likely be a driver or a crossover component in your speakers.  

Ahhhh, thanks, @carlsbad2  , that would make sense. 

but you said the voltage discrepancies followed the tubes.  Does the discrepancy at different frequencies follow the tubes?

I didn't actually go that far.  I do have a couple of pairs of 6sn7s that meausre almost dead nuts even, so I could spend some time and what the sound level meter said with them in there at the two different frequencies.

I know my question was probably confusing, although I tyed to spell it out as best I could, but what I did first was to use the AC vm to adjust the "input level" knobs until I was reading equal AC volts at the speaker terminals (with the 315 Hz tone).  It was then that I had the bright idea to see what the actual speaker output at my listening position, from each spaker, was.

Thanks.

@carlsbad2 , sincere thanks for your input. I did not play with different input tubes, but I do actually have a HT center channl version of the 805 Matrix that I have not used for years, so, even though I figured it was a fool’s errand, and therefore I was the right man for the job, I did play musical tweeters (actually with both speakers). To not make a short story long, I am pretty sure my wacky readings with 315 Hz were due to operator error.

I discovered that the readings with 315 kHz would change, for example, depending on where I was standing when I was trying to read the meter, and even depending upon where my head was when I was trying to read the meter. I won’t go on & on about how I tryed and failed to establish a reference point for myself and the meter at 315 Hz, but I did discover that the meter was way more stable (less subject to fluctuations due to my own positioning) at 20Hz to 20kHz and with the "input level" knob adjusted to compensate for the weaker preamp balanced input tube AC voltage at the speaker terminal, using pretty stable readings with the 20Hz to 20 kHz tone, the speakers had basically  the same readings from my listening position.  Meaning, that with the "input level" knob adjusted down (on the side I have the stronger tube on) to achieve equal AC voltage at the speaker terminals of both speakers, I also got nearly equal readings with the sound level meter.

So, much to do about nothing, but thank you for your input.

Going back to the untreated room - different tones reflect and build up in a room differently due to different wave lengths. Could it be that lower tones are being reflected and build up on one side or the other while higher tones are being absorbed? If room treatment is the issue, the fact that it changes depending on the frequency would be expected.  
 

I would find a couple different mono recordings, with a mix of highs and lows, play them and adjust until the image is centered. Howlin Wolf on one side of the spectrum and Peggy Lee w/ Goodman on the other should do the trick. 

@zazouswing  , thank you for your input and I believe that you are correct.  I also think that I was woried about nothing . . . I have a solid center image and I guess that is all I should have really been concerned about.  I don't know what led me down the sound level meter rabbit hole . . . there is no accounting for the stuff I sometimes get obsessed with.