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.