@MDB : Bigger speaker means lower frequency (a viola gives lower frequencies than a violin).
As for the very high frequencies: Usually, with high frequency loss, the loss gets worse the higher the frequency.
So, to make overtones at, say, 12 kHz audible, you would need a ridicoulus amount of gain at that frequency, which would result in distortion, clipping, feedback.
My advice: Use an aid that states “up to 10 kHz”, then this aid will be able to give real gain at 8 kHz, and with that, music will sound very much better than without the aids.
To get more performance, use headphones / earbuds in conjunction with a good equalizer. Try to reproduce the gain-curve of your aid (try not to equalize your loss, this will blow your ears!!!), this will give a sound not to far away from what you are used from your aids, but with better fidelity.
I really had those thoughts, too (what about 10 kHz and above?) but I must embrace the fact that I hear so much more with the aids in , even though they have no real gain above 8kHz, that this is still a large improvement!!!