Piano and Trumpet can sound bright

Yep, metal -dome tweeters CAN be brutal. I find that spkr. placement can alleviate some of this-the main offender IME is off-axis response. I recommend playing around with the amount of toe-in to find the least annoying positioning.
First reflections off of side walls can also play hell in this frequency range-try the mirror trick-put a lamp where the speaker is, and have a friend walk a mirror along the side wall- the point at which you can see the lamp from your listening position is the place to kill the reflection with an absorbent panel.

It must be said that both of those instruments are quite bright naturally, especially the trumpet. But many pianos are too, especially the Yamahas that are in wide use today.

the one time I owned Kimber 8TC I found it a bit thin and lacking bass, on Maggie speakers however not Celestion...

food for thought

Weren't you asking recently about a speaker change? It appears you didn't change out the speakers. Well... I believe at least a couple people, myself included, told you that the Celestion speakers were harsh in the treble as they are older metal domed tweeters.

You likely will find that as you increase the resolution of the system your speakers foibles will continue to reveal themselves. Time for a speaker upgrade.

The brand of cables also makes a huge difference in the nature of the treble. Trying some other cables may help.

Kimbers are great cables.....but then can be a bit forward, particularly the silver streaks. Substite a cable or two.. It should fix the problem.