Piano and Trumpet can sound bright

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.

Some of that zip may be diffraction from tweeter off the front baffle. You might try a large ring of 1/4" felt around the dome. This will slightly pad down the tweeter while improving coherance.

If you're up for a speaker change, you could certainly change that quality in the sound. However, you'll change pretty much everything else too. I'd try replacing the silver streaks with something that's all copper - there are tons of options.