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
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