Your first post is a bit hard to parse, so let's see if I can rephrase the question:
You added a curtain that covers your entire front wall that is closed when listening to music, but open when watching movies, as you have a screen behind it.
Prior to the curtain, you experienced a lot of echoes with just the screen and the bare walls, right?
Now, you are concerned that listening with the curtains closed, you might have too much absorption on that wall and 'closing in the sound'.
If I got that right, then here is my feedback:
Adding dampening of the high-frequencies (which is all that curtain does) on the front wall will indeed shift the soundstage and tonal balance of the system, but often times, it's a good thing, as depending on your room surfaces and treatments, that reduction in high frequencies from the rear waves let's you enjoy the direct sound much more.
The challenge is that since you only absorb the HF, the tonal balance can tilt towards a 'chestier' sound as the mid-bass and mid-range is more apparent. This can be addressed by adding something like a pair of Realtraps minitraps HF behind the curtain just behind each of your Summits. Their absorption of energy down to the 250Hz region will balance out the tilt caused by the curtain.
Now, the difference you hear with the curtains open vs closed is just the nature of the beast. With them open, you are letting the screen and the wall behind it 'reflect' any energy that hits them.
BTW- that's one reason why I have strong preference for Acoustically transparent screens, not only can one place the center channel directly behind it where it truly belongs, one can also treat the front wall with absorption as it's behind the screen.
Hope that helps.
