Looking at your system, I think its most likely that your speakers are the problem, and not the amp. B&W tweeters have a tendency to sound harsh. I have a lot of experience with them.
There are 2 ways to deal with this problem. The first is to get a different speaker. The second is to try and fix the problem indirectly by swapping out components and accessories to start off with, and then get new speakers.
There are 2 ways to deal with this problem. The first is to get a different speaker. The second is to try and fix the problem indirectly by swapping out components and accessories to start off with, and then get new speakers.