material for curtains? best absorbers?

Go to klone-audio for information and pictures that you may find interesting. I built some panels for my room and they improved the acoustics in my room tremendously. Good luck!

Many like wool or velour.

You can get a lot of reflection from glass, especially large picture windows. I have Pottery Barn curtains over mine and it is a noticeable improvement when listening at night with curtains closed. I would expect the denser, more tightly woven, and thicker materials would be best for having higher absorption.

If you are not covering windows I would suggest hanging wool area rugs instead of curtains.