Piano high notes very bright

I agree, pianos can vary widely in timbre. Yet there is another factor. Piano recordings typically are made with the microphones very close to the open lid of the piano. (Do you listen that way in person? I don’t.) One result is that recorded piano sound often is very bright. A producer or mastering engineer might adjust that brightness or use a microphone that minimizes it. Still, many recordings are too bright, leaving you to do something about it.

Your statement, "Ever since I rediscovered my hearing aids as an audio enhancement," suggests it is a hearing aid issue.  I don't have hearing aids or know much about them, but as one ages the high frequency hearing is lost first.  I wonder if your hearing aids are compensating for that by increasing gain at the higher frequencies.  I know some hearing aids can be customized to perform differently at different frequencies.  You might investigate yours and make the preferred adjustments.  

The piano is my favorite. I spent a long, long time trying to get my revealing system to reproduce the piano correctly - especially, the high notes - and especially via digital streaming. It finally happened with clean power and quality cabling. YMMV

- - -

As mentioned earlier, an untreated room can also be a big contributor to harsh HF.

Changing speaker positioning can reduce and even eliminate this problem. If your speakers are spread too far apart and are close to the side walls it can cause this issue due to how your speakers interact with room acoustics. Try moving the speakers closer together and away from side walls. Toe in direct to the listening chair to minimize the side wall reflections even further. Try it, it’s a free tweak. 

I have Phillips hearing aids, and I like my piano, it has to sound right. If I listen in normal mode, piano sounds just like you describe. If I turn on Hifi mode using the app, it calms down and sounds normal. Maybe post more about your equipment and room, and we could offer some advice on what to try.