Room Treatments added, with negative effects..

Room treatments can be a very tricky process, and without conducting a real-time analysis it's hard to know exactly what's required. The best suggestion I can offer is to try treating your room with the "LEDE", or "live-end, dead-end" approach. The LEDE approach usually calls for the acoustic treatments to be applied at the listener's end of the room, while leaving the speaker end (relatively) untreated. Try this and see if it helps. If not, then you will probably have to do a much more sophisticated analysis using a good test disk -- such as the one that Rives sells at his Web site.

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.