fabric for curtains to tame echo in my room.

I currently have a pair of Quad 988's with Thor Audio TPA-60
watt monblocks with a TA-2000 preamp. My room is 20 X 20 with 12 foot ceilings. The room has ceramic tile floors
and substantial glass areas. When people speak echos are quire prominent. My first recommendation is to install
thick drapes over my sliading glass doors and windows.
My question is:What fabric or material should line the drapes to achieve the best echo reducing response.
I have a similar set-up. Thor TPA30-TA1000 and Kharma 1.0 in a 24x24'great room with a 12' cath. ceiling.I first would put down a area rug ( with pad under ) in the listening area. Then I would hang long floor length, soft material, probably poly/cotton blend with sheers ( white transparent ) in between floor length curtains. This should ( as it did me ) help considerably. Also try ( wall ) hanging a tapestry or rug behind speakers. Experiment with a beach towel.Hopfully these tips will help. Good luck
Also try a valence - it will add an irregular layer which will help with dispersion.
I have vertical blinds over my patio door. They are made from fabric, and can be purchased at Home Depot or Lowes for about $70. I am considering buying another set and doubling up the fabric for each panel.

After you get the windows taken care of,look around for a rug that would look nice on your back wall between the speakers. I had a real nasty echo and this fixed me up without too much expense.I could hit pause on the CDP before and you could easily hear the slap back between the front and rear walls. It was also evident if you just stood in the middle of the room and spoke.... really distracting. Now it is gone. It was not too expensive to do either but of course that depends on where you buy the rug though : ) Look for the thinner variety and you can just use hardware to hang it just like you would drapes.