An amp will seldom overpower a speaker. In fact, blown speakers or poor speaker performance is usually the result of underpowering them. More power is always a good thing.
If the sound is not as bright as you would like, then it is the speaker that is not to your liking. Too much power from the amp would have nothing to do with it. But there is probably something else at play here causing the treble to not sound right. Most likely its your room, the furnishings in your room and the placement of the speakers relative to your listening position. I suggest you experiment with all of these factors first before you rush out to get new speakers.