It could be a number of things, but since I have had a similiar situation recently, I wanted to let you know that uncovered windows (at least in my room) were a disaster.
My system was sounding bright, and I recently bought auralex room treatment. I put it on all of the critical spots; my system was still harsh sounding. Then I saw a picture of someone's room on this forum who had a similiar room setup and size (smaller than your) and bingo! I had blinds over my windows and assumed that they would defuse the sound WRONG! Just like this gentleman, I put a folded blanket over the blinds and added some other treatment to just the window. I got the soft warm fuzzy sound back instead of the "I paid all this money for this kind of sound what the?"
Can't promise you it will work and I feel your pain, but it just takes a minute to listen to it before and after you throw a comforter over the window to see if it helps.
My system was sounding bright, and I recently bought auralex room treatment. I put it on all of the critical spots; my system was still harsh sounding. Then I saw a picture of someone's room on this forum who had a similiar room setup and size (smaller than your) and bingo! I had blinds over my windows and assumed that they would defuse the sound WRONG! Just like this gentleman, I put a folded blanket over the blinds and added some other treatment to just the window. I got the soft warm fuzzy sound back instead of the "I paid all this money for this kind of sound what the?"
Can't promise you it will work and I feel your pain, but it just takes a minute to listen to it before and after you throw a comforter over the window to see if it helps.