Room treatment...what is the goal?

The worst thing is to have an RT60 that varies all over the place (drastically different at different frequencies)- this will sound terrible.

Look at my virtual system for some measurement examples.

Most systems sound too bright in my experience; a rising top end gives the illusion of greater detail. Actually, there MAY BE greater detail in some area but others details will be canceled , the sound will not be in balance and it will be tiring in the long run. I have just been experimenting in my own room with varying number of panels, the correct sound will probably seem "soft" in the beginning but you will gradually notice that you can now hear details that were obscured before. The bass will also improve when you have the right amount of damping. It is possible to over damp, I use to have a full live end/dead end set up which improved a problematic room but which wasn't ideal. I would listen to the room as it is now for a while before I did anything else.

Good advice, Stanwal! You're exactly right.
I've had the same experience with various changes to my system, but especially with room treatment. What previously seemed like "sparkle" or "detail" turned out to be "hash."
Nemesis -- listen for a while, then, if you can, reverse the changes (go back to your original configuration) and you'll probably find out you prefer the well-planned and treated room improvements.

Agree with Stanwal and Ncarv.

I installed six 2'x4' acoustic panels on my sheetrock side walls to eliminate slap echo and ringing of higher frequencies. They certainly cured those problems, but the difference in sound made me worry I had overdamped. The panels were removable (but only slightly movable), so I experimented by taking down one panel at a time. There was a marginal improvement with only 4 panels, but for the present I am using all 6.

I also compensated for the lower sound (noise?) level by turning up the volume of the system. I found I could play at volume levels that would have been painful without those sound panels, and the sound is more satisfying simply because it is fuller. The sound is also better balanced because the high frequency noise no longer masks the bass and mid-range. I also understand that pre-amps sound better running at higher volume because of less resistance in the signal path.

I listen almost exclusively to classical music and attend many orchestral and chamber concerts. Since installing the sound panels, I have tried to compare the liveliness of various venues with my listening room. Disney Hall in LA is supposed to be a "live" hall (tending toward the bright side). However, I never detect anything like a persistent brightness. All I hear is more detail and no dullness in the sound. I think overdamped, dull sound results from eliminating overtones, not reverberation.

The objective is to eliminate echos, reverberations, vibrations, resonances. I think most agree, those phenomena, will interfere with enjoying your music. However the goal must be to achieve the sound you like that you prefer only. If it sounds dead to you I accept that it sounds dead. If you want more sparkle then ask your acoustical designer to advise you about removing some part of the sound dampening, but proceed slowly or you might rebound to the upper end tilt you didn't like in the first place. If your designer is arrogant and tells you to like it because he or she knows what is correct, then ask another expert, or experiment on your own.In any event don't suffer because of another persons taste or measurments.