Grimace,
Problem - It sounds like you're hearing flutter echo at the listening position that is attenuated when you place cushions on the front wall.
Constraints - WAF, cost, minimal visual impact
Possible Solutions - Flutter echos are most easily heard in the >1kHz frequency range which is easily dealt with. To kill a flutter echo you will need to treat one (or both) of the two parallel wall surfaces, which in your case could be the front and/or back wall. I'd also stay away from adding more absorption as you already have plenty with carpeting and lots of stuffed furniture - adding more absorption will move you closer to a dead sounding room and likely something to avoid. So, here are some suggestions that use diffusion or reflection:
(1.) Add texture to the wall's surface to break up the flutter echos with say a shelf with books or assorted nick nacks on it. This option should pass all contraints.
(2.) Add bass traps with a minimum of 4"-6" depth with a 6" air space behind it BUT WITH a hard surface that faces into the room so as to avoid mid/high freq absorption. Place it in the corners where all room modes accummulate. You can take a 1/8" thick piece of plywood and stain it to match the room and curve it to create a polyfuser (or sometimes called a hemi-cylindrical) diffuser. Or take a 16" or 24" diameter Sonotube used for creating concrete posts and cut it in half to make a 180degree arc and stuff the interior with fibreglass. Cover the curved surface with wood veneer that comes with glue on its back side which is easily applied with a hot iron. Stain or polyurethane the wood veneer to match your room's interior and it'll look like art or an architectural column of sorts. Should pass the cost constraint and depending on taste may also pass the WAF and visual test.
(3.) you could use a diffuser like a QRD or Skyline that when finished in nice wood and stained could look like art work and possibly pass the WAF test. While these do offer some low freq absorption they aren't bass traps so if you want the benefits of bass traps similar to the polyfuser idea above then you might consider building a 'bench' beneath and between the 2 front windows and putting fiberglass in garbage bags (to prevent breathing in air born fibres) in its interior and using 1/8" wood to allow low frequencies to pass through it to the fiberglass.
I'd stay away from using plants as a diffuser - while their irregular shape seems like a good way to scatter sound, the thin leaves can only affect tiny wavelengths of very high frequencies so they aren't effective, unless you're a bat! You could stick a plant in front of something else to help hide the thing behind it, but for diffusion plants can't offer effectiveness over a broad enough freq spectrum.
So to sum up, I'd suggest breaking up the wall's flat surface with diffusion (skyline, QRD, hemi-cylindrical) and adding bass traps but with a hard enough surface to avoid more MH/HF absorption. Hope this is of some help . . .
Problem - It sounds like you're hearing flutter echo at the listening position that is attenuated when you place cushions on the front wall.
Constraints - WAF, cost, minimal visual impact
Possible Solutions - Flutter echos are most easily heard in the >1kHz frequency range which is easily dealt with. To kill a flutter echo you will need to treat one (or both) of the two parallel wall surfaces, which in your case could be the front and/or back wall. I'd also stay away from adding more absorption as you already have plenty with carpeting and lots of stuffed furniture - adding more absorption will move you closer to a dead sounding room and likely something to avoid. So, here are some suggestions that use diffusion or reflection:
(1.) Add texture to the wall's surface to break up the flutter echos with say a shelf with books or assorted nick nacks on it. This option should pass all contraints.
(2.) Add bass traps with a minimum of 4"-6" depth with a 6" air space behind it BUT WITH a hard surface that faces into the room so as to avoid mid/high freq absorption. Place it in the corners where all room modes accummulate. You can take a 1/8" thick piece of plywood and stain it to match the room and curve it to create a polyfuser (or sometimes called a hemi-cylindrical) diffuser. Or take a 16" or 24" diameter Sonotube used for creating concrete posts and cut it in half to make a 180degree arc and stuff the interior with fibreglass. Cover the curved surface with wood veneer that comes with glue on its back side which is easily applied with a hot iron. Stain or polyurethane the wood veneer to match your room's interior and it'll look like art or an architectural column of sorts. Should pass the cost constraint and depending on taste may also pass the WAF and visual test.
(3.) you could use a diffuser like a QRD or Skyline that when finished in nice wood and stained could look like art work and possibly pass the WAF test. While these do offer some low freq absorption they aren't bass traps so if you want the benefits of bass traps similar to the polyfuser idea above then you might consider building a 'bench' beneath and between the 2 front windows and putting fiberglass in garbage bags (to prevent breathing in air born fibres) in its interior and using 1/8" wood to allow low frequencies to pass through it to the fiberglass.
I'd stay away from using plants as a diffuser - while their irregular shape seems like a good way to scatter sound, the thin leaves can only affect tiny wavelengths of very high frequencies so they aren't effective, unless you're a bat! You could stick a plant in front of something else to help hide the thing behind it, but for diffusion plants can't offer effectiveness over a broad enough freq spectrum.
So to sum up, I'd suggest breaking up the wall's flat surface with diffusion (skyline, QRD, hemi-cylindrical) and adding bass traps but with a hard enough surface to avoid more MH/HF absorption. Hope this is of some help . . .