Vinyl(the other kind)

Actually they made a huge difference in my room. Without any blinds it was truly horrible, no focus, no snap, bass was booming. Three quarter shut angled up truly worked wonders.
Does not help having a tiled floor either but large rugs and 3 large squishy sofas helped there as well

My whole listening room is majority glass panels.
I have vinyl blinds on all walls, trial and error has proven that approx 3/ 4 shut has given best results.
Still a shite listening room though....