How to Solve High-Frequency Suckout in Room?

After upgrading my system including speakers, I'm noticing with more upper frequency detail, that the right channel has some degree of missing high frequencies.  I've confirmed it is my room by swapping speakers, swapping cables for left / right, and of course the cables are all in phase.

My room is quite large, open concept, but my system is to one side of the open area.  Ceilings are vaulted and are 12ft at highest point. The speakers are not near any corners, due to a jut-out on the right side and the other end being completely open. However, there is a partial wall on my right side that has no treatment on it that extends up to 12ft, from the listening position.  This wall starts 3.5 feet in front of the right speaker (about 1.5 Ft to the right of the right speaker) and continues to behind the listening position. 

I've tried putting pillows against the right wall and thought it may have made the problem worse?  There is no wall on the left side, it is completely open.  Does this make sense that there is missing high frequency on the right side, where the wall is?  And, is there anything I can do to fix this?  I will attempt to draw the setup but I'm guessing the alignment will mess up when I post this! 


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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. 

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