I had on my to-do list to replace a curtain on a window that is maybe 5 feet behind and left of my mixing position with something that would be at least a little more absorbent. I took room response measurements with the existing curtain open, closed, the blinds open and closed, and with a bass trap stuck in front of it. Absolutely no difference. So I'm saving some money and letting light into my studio.
Also, the above mentioned bass trap was in a corner that was problematic in terms of keeping the room functional for anything other than a studio, e.g. storing crap, and removing it from that corner made no difference in measured response. It is now unemployed and living in my garage.