Congrats on the purchase Joel, the M60s are a great speaker. Its nice to hear that the M60 is on par with the Imagine series.
Thin fabric will not do much for you acoustically. From (not so good) memory, 2" acoustic pannels will start to make a difference above 500Hz or so and 4" panels will reach down to around 100Hz. leaving a space up to the thickness of the pannel behind the pannel will increase its effectiveness.
I would not do any treatments until you have your system set up and have listened to it for a while. It took me 2-3 months to be able to effectively separate out room effects from what the speaker was producing. A lot of rooms get some absorbtion and diffusion from furniture, sheetrock wall, curtains etc so don't automatically assume you need to treat the room a lot.
It does look like you have quite a bit of bare wall though.
FWIW, a thick rug on the floor between your seating and the speakers will also help tame a lively room.
I am looking forward to your first post after you get the surrounds set up.
