I recently got pretty tired of dealing with harsh and strident highs with my system. In my case, it was pretty easy to narrow the cause down to room resonances in my new house, since I had recently moved and had not made any equipment changes. Before you spend a lot of money on equipment, you might want to rule out the room as a cause of the harsh sound. If you have something that will absorb or diffuse sound to put in the corners it should give you a clue if this is what is going on. In my room I put about $110 into some Auralex studiofoam and used it to deaden the corners and it made a world of difference. Lots of dealers that sell musical instruments carry this stuff for people who are setting up recording studios.
If you have a really good audio dealer near by, talk to him about your problem. He may let you borrow a demo amp to try for a few days. If you try another amp and still have problems, I'd keep working on the room.
If you have a really good audio dealer near by, talk to him about your problem. He may let you borrow a demo amp to try for a few days. If you try another amp and still have problems, I'd keep working on the room.