I bet for a lot of audiophiles it is a sweatspot indeed...sorry, I couldn't help it :-)
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Your overall low ceiling height might be the big culprit here. Have you tried playing with the feet adjusters (spikes or whatever) to position the tweeters at more of a downward angle rather than level to your listening position? Also toe in or tow out? My sound stage on good recordings ranges from 1/2 way up a 8' wall to the ceiling and I get down right upset if things like vocals are not at least 6~7 feet off the floor. On some recordings it almost washes over top of my listening position.
PS if you have an AudiogoN membership, if you log in to see your stuff and stay logged in, THEN you can edit your posts. i do not always do it, but it is nice to know. And YES if you log in a little later you CAN go back and edit your post for at least a little while. (I do not know what the time limit is on editing, but it is not more than a few hours for sertain.)
How far are you from the face of the speakers?
You say,"When seated in the sweet spot, all the highs seem to be comming from where the wall meets the ceiling."
That would be 'behind' the speakers at the joining of the 'rear wall' is that right?
If so, that strikes me as odd.
I think, to start, I'd try listening, more nearfield...as a matter of self defense.
Next experiment, this might sound really dumb, but, note how some concert hall, stage areas use vertical hanging's from the upper part of the stage.
Try, for a point of reference, hanging something absorptive from the ceiling (a towel for experimentation, if nothing else).
Put that hanging, parallel to the rear wall--starting a foot from the face of the speakers, then further and so on.
Maybe a 'team' sports banner would help a bit, but you've not got much room--I know I couldn't walk in that room with anything more than 5" or so hanging down.
This is very approximate, but we don't know enough yet.
If you'd like to discuss this, you can write me at my personal email, email@example.com. I'll be glad to give my phone number to talk more.
We can solve this somehow...no accoustic problem is too much when you're willing to experiment.
I finally fixed this problem by stripping the ceiling down to the joists. The ceiling effects the system in way I did not think possible. I called several people who are in to acoustic treatment and they said my problem was very weird indeed. No shit. I now need to figure out what to put back in.
With the limited information provided, I guess the undue soundstage height is due to disproportionately strong ceiling reflection. Due to low ceiling height, the reflected sound off the ceiling arrives at the ears very close to the direct sound of the speakers. The ears tend to integrate early reflection with the direct sound. This leads to perceived sound source broadening.
Just traps or absorbers would decrease the amplitude of the reflected impulse but would not spread out the reflected sound over time. The ceiling would still be very audible.
Combo tools (abfusors, skyline, hemifusors), or diffusors tend to help a lot in this situation as it would decrease the amplitude of the reflected and also spread out the reflection over the time domain. Having a softer, later reflected impulse tends to fool the brain into perceiving a boundary further away. I would think hemifusors or low profile skyline wound be excellent as they occupy less space and the slightly effective narrower bandwidth may limits beaming since listener is still close to the diffusor.
The brain composes the soundstage by reflection off all boundaries. If you have absorptive floor ( thick carpet), it wound further expose the soundstage broadening effect from the ceiling. Changing to wood floor would strengthen the reflection of the floor and would counteract the effect from the ceiling to some degree.
Changing the dispersion of the midrange/tweeter may also help. If it is an option, ribbon tweeters have broad horizontal dispersion but very limited vertical dispersion. This may yield a more stable/precise soundstage on the vertical plane but maintains a wide stage on the horizontal plane. Other ways including install a waveguide or using felt to limit vertical dispersion.