I'm thinking changing the crossover probably won't really help with the higher frequencies and harshness you describe, as that would not be around the lower frequency ranges.
Possibly check your speaker wires to make sure you have + to + and - to -. As the others have stated, you want it up close to or beyond the shelf lip so you don't get any unwanted reflections. What surround mode are you listening in, I assume one of the dolby or dts modes, and not one of the special modes. Now that it is on a shelf, do you have it angled upward by flipping the speaker over? Have you tried listening close to see if possibly one of the drivers is bad...
just a few thoughts.
