Derek,
> Possibly the cathedral ceiling is the culprit? <
A cathedral ceiling is a problem because it tends to focus the sound under the peak. Focusing is the opposite of diffusion. I hung MiniTraps under the ceiling peak in my home studio:
My living room home theater also has a peaked ceiling, so I treated that the same way too.
> Is there an area I should focus on or is it simply about coverage (like bass traps)? <
Yes, it's about coverage, but there is a method you can use. Besides reducing overall ambience and excess reverb, you also want to avoid flutter echo between parallel surfaces. So this means you should treat opposing walls. But don't treat one entire wall and leave the wall opposite bare. Better is to treat uniformly, such as a checkerboard pattern or stripes. The goal is to spread the absorption evenly around the room.
--Ethan
