How to Solve High-Frequency Suckout in Room?

Okay, so I spent half my Saturday totally tearing apart my living room, removing the couch, and bringing in large cushions, moving blankets, and cardboard (my speaker boxes), to play with various configurations.  I also tried the "excessive toe-in" configuration where you have the tweeter's line of fire cross over a few feet in front of the listening position.  I tried moving my entire system extremely to the left.  I also removed my old speakers from the area.

While some things helped a bit (the thing that helped the most was moving the entire system to the left), the issue ALWAYS remained, no matter what I did.  I even tried changing the right speaker so that it was asymmetrically positioned to the left speaker.  I also compared the same tracks with a decent ($500 Sony WH1000 which are great) pair of headphones just to make sure I wasn't imagining things.  Headphones reminded me of what a perfect stereo image is supposed to be.

So, after all that I realized I had accomplished two things: 1) I don't think my room is to blame, and 2) I found I had a ton of crud under my couch which I cleaned up.

Finally, I went back to wondering about my amp.  I swapped the speaker cables on the amp end only - moving the right cable to the left amp terminal and the left cable to the right amp terminal.  And this time, I REALLY listened.  I realized that the problem did in fact move to the left side, from the right!

With this in mind, I now suspect my Gryphon Diablo 300 amp has an issue.  I'm using the internal Gryphon DAC module, so it's possible that is the culprit and not the amp. 

Has anyone heard of something like this happening with an amp before?  What could cause it?  Frustrating as I adore the sound my system is producing other than the lack of stereo image.  On the other hand, I feel like I'm getting closer, in that I'm getting closer to isolating the cause of my issue.

On a related front:  Why sort of meter should I consider investing in to measure the frequency response of my channels?  And what test track should I use for measurement, to prove beyond a doubt it's not in the source material? If I go back to my retailer/Gryphon with this issue, I feel it would be good to be armed with some "proof".

OP,

Thanks for the photos… I like the one with your significant other (?) peering out> Nice system. 
 

With what you have done, I would also would be at wits end. Happily it sounds like you are working to root cause… that it is it is a component problem. That makes sense. Because, yes, as you say room treatments can dial things in… this problem doesn’t seem to be a room problem. 
 

Easy test without buying anything would be download and run a 1khz and/or 10khz test signal through one channel at a time and use a SPL meter on your phone. Make sure you don’t change the volume when switching. The right channel is the one you suspect so I would test the left first get up to 85db with your phone 1 meter from the center axis of the tweeter and mid. Then try the right channel 1 meter from center axis and see what db you get. It’s not a perfect test but it should show if there is a big variation. Should be able to  hear if there is a big variation. 

@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?