If blankets provided an improvement as you say in your post, that indicates you are looking for absorption. Absorption is what acoustic curtains would give you. Curtains will not be thick enough to absorb low frequencies. So, they will reduce some of the high frequency glare that I suspect you are getting with all that glass.
As for what rating I assume you’re talking about NRC ratings. They usually vary from 0 to 1 or so. The higher the NRC number, the more it will absorb. However, a fabric’s absorption will vary with frequency. The easy answer is the higher the better.
A common confusion is NRC v STC. STC is about sound transmission. It measures how much sound gets through. It is used to make things quieter as opposed to absorption. For a fabric it’s likely true that the higher the STC, the higher the NRC.
The bottom line is you want a thick fabric, as heavy as you can deal with. That will absorb the most. You room has lots of glass so I expect it would be difficult to over-absorb with curtains. With all of Ivan’s experience with home theater I suspect he would be a good source for where to buy.
And......if I come across as an expert, I am not. I did a lot of reading as I was trying to fix the acoustic issues with my small listening room. So, this is just some stuff I learned doing research and talking to people.
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