@Volusiano
Thanks for your explanation it is indeed important for all music addicted to know about these differences.
So if I am talking about my special interest I have to talk about “frequency compression” and not just talk about compression as you and others could mean something else and this will be the confusion. I understand that and will edit my first post here by adding “frequency”. As this is what I would like to emphasize and I saw in rereading my first comment that I omitted this.
Nice to hear you also play the piano, one of my favorite instruments with a wide spectrum of available frequencies. The important thing with a piano is the frequencies are fixed so if the instrument is in tune you don’t have to worry very much. Even someone with HA’s frequency compression on could probably accompany and has the opportunity to establish you are all together more or less in tune except for notes played out of this piano bandwidth! (So not talking about gain compression here 
). Our brains are doing a great job in artificial intelligence, who invented that.
But at the other hand the classic piano is somewhat limited in musicality as it lacks for instance the possibility of vibrato and crescendo. But that’s another topic.
For me, I play a string instrument (cello) myself, this frequency compression control makes it for me impossible to play with. (I use it though for my speech- understanding because it luckily and surely brings up my word recognition rate a lot and I am happy and lucky it can do the trick to allow me to communicate with my environment not only by vision).
Actually this musical topic is for me of great interest as not only being interested in music but also in medicine. And I read as much as possible about high tech and future of HA’s. I also understand that it is not possible for a human being to know and understand everything nor be flawless. And no one ever will give me back my former absolute hearing I am still longing for but enjoyed in former years.
I’ll try to simply explain what the problem of frequency compression for me is. Musical instruments I accompany, when I do not play solo, are mostly playing in other frequency bands. You will always try to play in tune and try to find out who could be a bit out of tune and what to do about it. It is a precise teamwork.
Next to that, depending on the historical data of the pieces you want to perform, you’ll have to tune if not solo with the other string- instruments at different pitch.
And to even make it more complicated there are changes in each hand made instrument, the pitch will depend on bow strength, place of bow on the string, kind of string used, vibrato, air and instrument temperature etc. So I have to make a decision about placing my fingers on the only right estimated place of the fingerboard for each intended note. You lucky and happy piano- players, you’ll be able to always find the right note on the exact same place and it will be always in tune I hope.
No helping frets or other possibilities for me to show the right direction of an intune note. Only thing available is my experience, my muscular memory and my ears and also passed body- vibration to estimate the right place to put my fingers. For all other instruments then my cello and apart from the piano they have their own specific tools to be able to graduate their frequencies and try to stay in tune.
What I would like to say for the ones interested in music and HA’s is that you have to be clear that some problems cannot be solved. Even if you have big pockets and can effort the most expensive HA’s.
