Oh, and for the theme: The theme would be a bit clearer if the game was bigger.
For example, you could have an upgrade that boosts your damage when you are low but risks another upgrade not working (because certain instruments fade out).
Additionally, you could use your faster shooting on your right mouse button to gain a momentary advantage, but it could also affect your upgrades ( and music, visuals, and small things like bullet speed, which can also change based on how many instruments are playing).
Visuals are also just a small detail. If you notice, at smaller percentage levels, some post-processing effects are reduced. Also, at higher health, you can see the bloom also being in sync with the beat, which results in pulsating (you can notice it on the UI).
As you said, more explanation would have been great. Again, thanks for the feedback!