Here's something I did one time that was very effective. If you want to experiment, it's pretty easy to do. Afterwards, you can add the fiberglass to your attic, or figure out how to hide or make it look acceptable.
In two room corners near the speakers, I tacked up a piece of standard 3.5" fiberglass with the kraft paper side out, from floor to ceiling leaving a small airspace. It made a huge difference in the midbass / midrange. I had planned to make a cover later, but never did, so I later took it down. If your room is big, try taping two thicker pieces together to form a larger airspace in the corner. It will tend to be round not flat, and will aid dispersion as well as absorbtion.
What I thought would work, but haven't tried is similar. Using the same 3.5" fiberglass, tape several pieces together with duct tape on the paper, and form it into a big tube about 15" to 30" in diameter, leaving just an airspace in the middle. Seal the ends with wood, or more fiberglass and tape.