acoustic placement/small bright room

Small rooms require much more damping than larger rooms. You actually can't have as long a reverberation time in small room as in large room without this ill effect you've experienced. This is because for the same reverberation time there are many more reflections and paths that cross the listener. The result is a very muddied and unclear sound. It's also not just in the high frequency--that's where you've heard it the worst, but it carries down in to the midrange as well. Then what can be worse is what happens to the bass--you can get severe bass build up in small rooms. If your dimensions are good--in the right ratios, then your problem will be minimized, but if they aren't, then you will have another issue to deal with.

My guess is that your drapes are not overkill, but they probably have a low absorption coefficient relative to pressed fiberglass. Thus you will need a lot more quilt surface area than fiberglass.

Just got back to this thread. As to the poly fill, it depends on the loft. If it's a really thick quilt and the covering is natural fiber (cotton preferably), then it's probably doing a reasonable job. If it's polyester covering then it's going to reflect somewhat. Also, the loft is small, there is going to be much less absorption. Fiberglass is about the most efficient absorber, and the higher the density the more efficient it is.