Our new basement had a really nasty snap echo in one end that was really screwing up the sound of my SDA's. There is a boxed in support beam that creates a small rectangular area in the ceiling as seen in the attached pictures. It was annoying to talk in this end of the room much less listen to music.
I found a product called Corner Busters (http://www.echobusters.com/cornerbusters.html) that claims to correct such problems. I temporarily stuck throw pillows in the corners just to see if this would work and sure enough the pillows helped. The Corner Busters seemed like the ticket but they are $190 plus s/h for a 4 pack!
Plan B - I found a company called ATS Acoustics (atsacoustics.com) that sells all the stuff you need to make your own. I bought a sheet of 1" thick Owens Corning 705 for $18.80 + $9.94 shipping = $28.74. The local fabric store had a good selection of canvas and I picked up 2 yards for around $15. Home Depot had 3m Super 77 spray adhesive for around $14. I built 4 for less than $60! Owens Corning 703 is cheaper and it may have been just as good but I only needed one sheet.
They work - No more echo!!!! The annoying mid/hi frequency echo is gone and the bass sounds much tighter.