How to Solve High-Frequency Suckout in Room?

I did a quick test with my multimeter and will need to re-try the test tomorrow.  Unfortunately I don't have the house to myself and people are sleeping!  I do need to have my speakers connected to do this test right?  I did find that the right side was consistently .001mv lower than the left channel across many but not all frequencies, when my amp was set to it's lowest volume setting - probably meaningless.  Will try at higher volumes tomorrow.

Okay, I tried testing a series of frequency tones without any load / speakers. The voltage of the right channel seems to be slightly higher than the left channel, when playing tones from 800Hz or so through to 12,000 Hz. The difference in voltage is greatest between 4,000 Hz and 6,000 Hz with the maximum difference in voltage between left and right channels being 7mV at 5,000Hz. Across the 800Hz-12,000Hz band, the right channel is always tad HIGHER in voltage than the left side. Outside of this band, the voltages between channels are identical.

Is this difference enough to matter? Interestingly, my ears tell me the problem is centralized around the upper mid range, so that corresponds with where I measured the greatest variance in voltage between channels, at 5,000Hz. But, given the missing frequencies on the right, I would have expected to see LOWER voltages on the right, not higher! Hmmm.

I had my amp set to a moderately high volume setting (20 on the Diablo 300) which is which is about 3-5 higher than my typical listening volume. Here’s the data, with the frequencies with the largest variance between left and right highlighted in bold and underlined:

250 Hz L 694mV R 694mV

400 Hz L 694mV R 694mV

800 Hz L 695mV R 696mV

1000 Hz L 690mV R 692mV

1250 Hz L 684mV R 686mV

2500 Hz L 647mV R 652mV

3150 Hz L 627mV R 633mV

4000 Hz L 600mV R 606mV

5000 Hz L 567mV R 574mV

6000 Hz L 533mV R 539mV

7000 Hz L 497mV R 502mV

8000 Hz L 459mV R 463mV

9000 Hz L 419mV R 422mV

10000 Hz L 382mV R 384mV

11000 Hz L 349mV R 350mV

12000 Hz L 319mV R 319mV

13000 Hz L 287mV R 287mV

14000 Hz L 252mV R 252mV

Is this enough data to tell Gryphon I have an issue?  Or are these differences in voltages between left and right too small to matter?  If the differences too small to matter, well I do find it a bit coincidental that the largest variances in voltages are in the same frequencies where my ear hears the problem (centralized around upper mids).

Play the  1 kHz test tone into  speakers don't play so loudly that it damages your speakers. Set your voltmeter for AC Volts. Measure the volts across the speaker terminals. Power is: E²/R where E = volts RMS and R is resistance of load. Both channels should be the same at the same volume setting, maybe slight variation. 

Your left and right are perfectly matched. No issues

I thought there had to be a load on the amp to measure this  with multimeter but I might be wrong.