Changed Speaker Placement NOW BOOMY

12' distance should give resonance at 48Hz while 26' span amplifies 22Hz

F= 1150/(2xd) where 2xd is twice the distance (wave has to travel forth and back to add in the same phase).

Timlub - Yes, corners are the worst (I have window in one).
Damping materials work effectively to about 100Hz at 2" thickness. Further down it requires going to 4" panels (dense fiberglass) and positioning them 1/4 wave from the wall. I'm in process in building 2" panels (dense fiberglass material) but have to take care of corners as well. A lot of work and it doesn't come cheap. In addition there is WAF.

Timlub- are you talking about corner traps? For the walls I have thin wood (pegboard like) 2'x4' boards and 2'x4'x2" high density fiberglass panels to glue on and wrap in fabric. I'm placing 8 panels on one wall and 4 panels on another (planning to buy another 6 - 18 total).

When I bought panels salesman showed me the room with walls made of similar material and effect was incredible. I could hear his voice so pure and coming directly from his mouth. I know that acoustically dead room is bad for music but I'm very, very far from that. I have a lot of reverberation in 15'x25' room that has cathedral ceiling starting at 12' (a lot of walls). Thinner materials like fabrics or blankets don't do any good. If you place such material if front of you and can still hear thru it the it is no good (won't stop wall reflection)

Timlub - There is a pretty good article here : http://www.ethanwiner.com/acoustics.html

It shows that my material (John Manville 817) has two times higher absorption coefficient at 125Hz than lighter 814 material (similar to common Owens Corning). It is still pretty bad for lower frequencies unless I'll go 4". Problem then is cost and WAF.

I'm not sure if reflections come from vaulted area. I think it is mostly from tall walls. I can hear echo, when clapping hands and also get better imaging at lower sound levels (echo dies faster). I had problems with previous speakers (Paradigm) resonating on upper bass frequency that was most likely multiple of bass refleks tuning. My Hyperions don't do that (why?).

"I've never sat in front of Hyperions, but they look great."

Timlub, you make me feel better. The only thing I didn't like about Hyperions was black piano finish and coffin like shape. Now it is slowly growing on me. I even realized that possible scratches on black piano finish can be easily repaired by piano repair/restoration shop while scratches on wood veneer are almost impossible to erase.

Josh358 - That's exactly my situation but EQ won't help me getting rid of echo. What happens at lower frequencies is also echo in a sense that reverberation changes sound (bass attack, decay etc.)