The problem with curtains is they absorb only high frequencies. Proper room treatment addresses all frequencies more or less the same. Otherwise the room ends up sounding too dead and too boomy at the same time. Curtains are great for appearance, but they work best with proper acoustic panels behind them. This short article is mainly about home recording, but all the same principles apply to hi-fi and home theater acoustics too:
I would further the idea of balance (treating frequencies equally). Curtains could be balancing if you had say a reflective floor and/or other reflective surfaces to counter the high end absorption provided by curtains.
So what change(s) would constitute balancing or unbalancing depend on your initial conditions.
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