Piano and Trumpet can sound bright

To fight aluminum tweeters-try different tubes? What do you have in there now?

I would have to say the speakers (aluminum tweeters). To me speakers make the biggest difference. They all have a very specific sound. It's what sound you prefer.

That being said I have a tube integrated and some tubes are definitely brighter than others. Not by that much but my Ei KT90's are very bright in the upper mids and highs.

Just my opinion

What's your room like? Treated, lots of hard surfaces?

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.