Sound Absorbing Drapes

I have a couple suggestions. This sounds like a big project that will involve hundreds of dollars and dozens of hours.

1. I highly recommend that you run Room EQ Wizard to get a baseline of where you are at before adding more treatment. The software is free and the calibrated mic is under $200.
2. Look at companies that provide sound absorbing curtains. There is a lot of detailed information about the materials and construction they use. It may give you some ideas about making your own or it may lead you to purchase a manufactured product.
3. Take the advice of @ghdprentice and experiment with heavy blankets before making curtains. Sound absorption is tricky and you may not like the result.

Some more notes about my current setup:

• My room is 13’ W x 18’ L
• My speakers are pulled out from the front wall 50", I can’t go any further (WAF)
• My speakers are Martin Logan electrostats
• I currently 2x4x4" thick sound absorbing panels behind each speaker

My sound stage is wonderful, but I would love to switch from panels to heavy fabric curtains on the front wall, if possible.  I think removing the panels would tremendously improve the aesthetic of the room - BUT, I would only do this if the soundstage were not adversely affected.

I guess my real question is - which would perform better - 4" thick panels, or thick drapes??  I am even open to putting two-inch thick panels behind the drapes (hidden from view).

Thank you all for your comments and feedback on the matter.  I love this forum for these purposes.

The result off a methodology using materials that are wanting in how sound energy is managed will certainly not be appropriate to create a accurate experience of sound energy being managed by materials correct for the role.

Glass Wool such as Rockwool at a Slab density @35Kg/m3 where the Batt is slipped into a cotton covering wi introduce fit for purpose materials to trial with. 

Do create the space between inner face of the Batt and the Wall. The bounce back is and important sound energy management. 

Reserve a bedding material to keep a Winter Chill more comforting to bear in the Winter

@hikerneil 

You should check out cellular blinds.

ghdprentice +1

ghdprentice is correct in that heavy absorption behind a planar speaker will overly deaden the rear wave and seriously dilute what’s great about a planar design. 
Many years ago in my first apartment I bought my first real pair of speakers, Martin-Logan Sequel II’s, a hybrid design with a 48” x 12” electrostatic panel and a 10” woofer in a sealed box. My apartment was of generous size with a soaring vaulted ceiling. But I could only pull the Logans maybe 2-3’ from the wall behind them which noticeably blurred the soundstage and caused a bit of midrange glare.  After some experimentation, I found some handmade tapestries in a shop on an American Indian reservation in northern Michigan. Though they were sized like a throw rug, about 5’ x 4’, they were meant as wall hangings. These were woven from knotted soft cotton roping into a strikingly original brightly colored design. The knotty texture provided some diffusion and the soft cotton offered a bit of absorption. Hanging one behind each ‘stat panel involved a simple pair of finishing nails into the wall. These original pieces of artwork brightened up the room and brought the Sequels into focus.