Room Treatments added, with negative effects..

Not nearly enuf info in your post to be helpful to you, but FWIW my order of priority in setting up diffusion (not deadening) materiels for dynamic speakers is sidewall reflection points first, behind the listener 2d and behind the speakers last. I'd remove the panels you now have, piece meal in that order and see what happens. Good luck.

Acoustics can be tricky. Actual acoustical measurements can make a tremendous difference in designing a room, but even without them if the room is basic (rectangular and no openings) then basic math and experience can yield a good result. SD's advice is also good, but there is a small correction. LEDE originated from studio control rooms. The live end was behind the listener and the dead end behind the speakers. This is good for control rooms--but bad for listening rooms. The LEDE method that SD describes is actually the inverse of the original LEDE--but it is the one that I agree with for audiophile listening rooms. It allows short reverberation times near the speakers which adds to ambiance and it controls the long reverberation times behind the listener. I could go into a very lengthy discussion on the philosophy of this, but I think most people would fall aspleep.

The other thing is that applying only one type of acoustical treatment, in your case high frequency absorption, generally does not yield good results. You need to judiciously deal with the entire range of frequencies from low, mid, to high. I'm not really surprised that things sounded dull and calapsed after applying this material--you need to do some other things as well most likely.

Our listening room, a basic tutorial on room acoustics, may provide you with some insite:
the listening room

Rather than putting them behind the speakers, try putting them on the side wall at the first reflection point. Since they are 24x24, I think 2 or at most 3 on each side should be enough. After that, put 2 or 3 in the middle of the front wall and 1 or 2 at the first reflection point on the ceiling.

take away the treatments behind your seat---put in diffraction. absorption behind the seat usually kills presence.

Thanks folks for the comments so far. Some interesting possibilities. Just to mention, while I can put some absorbtion material on one of the side walls, it won't be symetrical with the listening position and the speakers (they will need to be closer to the listening position, as there is a corridor in that area of the side wall). Also, the other wall is all glass door, with curtains between. I could attach the material to the appropriate part of the curtains, but that seems like stretching things a little. I guess I could try the ceiling as well.

Overall, from what I've heard it really seems like the panels have only deadened the sound - I somehow have my doubts about any benefit they're going to offer. I'll try removing them from behind the speaker, and then maybe try the opposite as well. I'll also try removing them and adding more to certain areas. It still seems like a big confusing mystery though. I wonder would the addition of some highly reflective surfaces (rather than absorbant surfaces) made a positive difference. Cheers,
Outlier