I have a wickedly unpleasant 800Hz spike in my living room. Major bass suckout as well. It's a peculiarly shaped room with ceiling which ranges from 12 to 20' and then a stairway which goes up to a landing which is 30' above the living room floor. Worse, there is a skylight which is a 3.5' x 3.5 'chimney' up to the slanted skylight. Acoustically, a nightmare.
So, I built one (building another) 7' x 4' framed treatment with 2" of rockwool behind a patterned fabric. I will try to post pictures.
I'm hoping this will tame the 800Hz. Ian suggested multiple subs for the bass problem. I have 1 EP600. Might be cheaper to get a new house.