Speaker recommendations

This is such a broad question I wouldn't know where to begin.  Other than suggest that you identify what it is about your existing speakers that you object to, and go out and audition some that don't have these characteristics and sound better to you.

Your NAD 375BEE puts out a very robust 150 wpc and its 4 Ohm rated.  That means there are literally hundreds of speakers, new or used, that it can properly drive.  Go out and listen to a few.  Good luck!

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.