How should I treat window behind listening chair?

Well, something is better than nothing. That glass behind you is too reflective. Light drapes might be just the thing. You might run some tests by hanging up light materiel and changing the density incrementally until you get the sound you want. Sheets, blankets etc.

I have a doorwall behind my listening position, and I use two EchoBuster Double Busters in front of it. You're obviously dealing with a window, which is different, but it might be worth contacting EchoBusters and seeing what they offer for that application in terms of diffusion/absorption.

Heavy honeycomb cellular shades is a reasonable compromise in many cases.

Rives -

Any chance the BLACKOUT material INSIDE some of the cellular shades helps versus non blackout material shades? Just ordered some and had wondered this.....

Thanks
Dan

I have drapes on 3 of the four walls of my listening room because it's a very small room and had annoying slap echo (clap your hands and hear it ring). I find the drapes hugely improve the sound.

However if your system sounds dull and lacking in treble detail the drapes will make it more so, as they affect high frequencies proportionally more.