Good Materials for Curtains

If you live in a city, I would use curtains that shield against RFI, something with a metal braid woven inside them.

If I may, I know the curtains won't soundproof. but why the companies claim that they're making soundproof curtains https://www.geekwrapped.com/guides/best-soundproof-curtains see reviews on them at the first time. 

It might knock down a bit of the upper frequencies but the mids and low end will go right through it.

I would suggest buying the heaviest natural material offered in the width and length you require. In back of them on the same or on an additional rod have a slightly smaller pair of wool blankets with with slits cut into the top that will go on the rod as a backing panel. You can conceal any appropriate material behind the chosen curtains so that it is inconspicuous to everyone but you and your significant other. The heavier the materials the more bass absorption you will experience. As always your mileage may vary. It may take some tweaking. Isn’t audio FUN!🤡

I think a lot of it depends on whether you’re naked or not whilst listening to your HiFi.

These curtains are meant to block outside noise more than provide a sound treatment for a room. Probably will do both, however.