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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@ghdprentice thanks; actually that is my daughter peeking out! She tends to do that and I don’t even notice...

At least component-wise, it has to be either my amp or the DAC module in my amp.  Can't see how a USB streamer would cause an imbalance of frequencies between channels!

I have a hunch it's more likely to be the amp, not sure why, but I could see it being the DAC possibly.  I am going to dig up my CD player and input it into my amp, to rule out or pinpoint the problem to the DAC module.  Also, I have an old Arcam A85 integrated amp tucked away that I haven't used in years, that I could compare with, but if I recall I think it only accepts banana terminations.

Thanks @djones51 , I will try that.  I'm not sure if the variation is "big" or not.  I am very sensitive to tonal imbalances, so it could in fact be very small.  Although my wife can immediately hear it as well.

Also, while it is blatantly noticeable on many tracks/artists/recordings, especially if there is not a lot of reverb in the recording, mostly being apparent in upper ranges of vocals (singers sound "fleshed out" in the upper ranges on the left but not the right), on many tracks the effect is very, very subtle.

@djones51 I will try your suggested "imperfect" test.  But I'm curious, what would the perfect test be?

Take it to the shop where they can put test equipment on it. If you had a multimeter you could run the 1khz test tone at 0db on the volume and see what the volts are on each side. 

I like the ides of trying  a different  smp. That should put the room or amp question  to rest one way or another.  For the try even a cheap set of banana  to spade converter s will do what you want. Some midfi low fi store should have them. 

@retiredfarmer I will try getting banana to spade converters; will go to my local shop this week. Then I can try my old amp. I'm not sure the issue will be detectable on lower-resolving equipment however. The issue was barely noticeable (but I did detect it) on my prior B&W speakers which were one model down. Seems that when the mid and upper frequencies are less transparent, the issue is harder to detect. Going to an Arcam A85 from a Gryphon Diablo 300 I expect will be far less resolving (But, I loved that Arcam for what it was, it was an unusual star among other Arcams at the time which were mostly junk).

@djones51 I have a multimeter and can try measuring the voltage on each side! Dumb question though: how do I play a 1kHz test tone, seeing as I normally stream through Roon/Tidal Hifi? My Gryphon dealer is actually not in the same city as me and I’m not sure my local shop is up for any sort of diagnostics.