I think usually good music reproduction is synonymous with being able to reproduce good bass, which is a critical (but not only) attribute for a lot of contemporary music genres. The HA receiver just doesn’t have the physical construction to enable this. There’s only 1 tiny little receiver that has to be optimized for the whole frequency range, which is no match for a speaker system with bass and mid range speakers and tweeter speakers all separate.
The other aspect is what we’ve discussed earlier in the thread, the lack of sufficient dynamic range on the input mic, as well as the A/D converter, to handle the loudness of musical instruments live. Digital tricks used to avoid distortions like dynamic compression also cause pumping which makes music more unnatural at times.
Then there’s also limitation on the HA mics themselves. Due to their tiny construction, I’m sure they’re no match compared to professional mics that are 100 times bigger in size and with much better pick up materials.
One of the things that is essential in HAs is feedback control. One of the way to minimize feedback is to run the feedback test throughout all the frequency channels and reduce the gain of the resonant frequencies that cause feedback. First off, the obvious trade-off here is the reduction in headroom of the frequency bands where gain reduction is necessary for feedback control. Secondly, the effect is worse for HAs with less number of frequency bands (like 8 or 16) because the gain reduction may be more widespread than it needs to be. HAs with more frequency bands like 24 of 48 channels suffer less of the headroom reduction effect because of the ability to fine tune the gain reduction to a much smaller frequency bandwidth the higher the number of channels there is.
Thirdly, like JustEd was saying, if the HA is running a program geared toward speech, then things done to promote speech like noise reduction and directional beam forming, sometimes increasing the gain on the frequency bands where speech resides, or reducing reverb or echos in an open room, can ruin the quality of the music. That’s why most HAs have a music program which essentially removes all of the processing geared toward speech so that you can hear the music sound unadultrated. But some of the modern HAs now may have auto listening environment detection and auto program transition, which may not necessarily be smart enough to know what is music and what is just a lot of noise, so there may be a good chance for the wrong (non-music) program to be selected when music is being played.
Listening to music in a noisy environment (like while driving a car) is another challenge. How would the HA be smart enough to know what is music and what is noise. If you switch to a music program then you’ll have to deal with the noise because most likely music programs don’t have noise reduction. If you switch to a car noise program then the noise reduction designed for it will stifle the music.
But direct streaming is one area where you can eliminate a lot of the variables that are limitations, like mic construction/sensitivity/dynamic range, digital processing geared toward speech, management of noise, etc. That’s why direct streaming should sound pretty darn good if you have the good source content and quality, and the right expectation about the limitation of the tiny little receiver that’s tasked to reproduce the sound in your ear.