I'm really going out on a limb here, but I'm gonna guess it might scare them a little or set them on edge, especially when there is a lot of bass. Here is my reasoning, and like i said, it is my reasoning, and am in no way claiming this to be fact in any way, shape, or form.
Ok, when you have your bass up really loud, you feel vibrations, or sound waves, as they travel through the air. I am guessing that as the sound waves hit the tank, they will cause some vibrations in the tank as well. Also, when you tap on your fish tank, it scares the fish, even when it isn't right where they are looking. Tapping on the glass is one way to create vibrations in the tank. Since loud music can cause things to vibrate, and vibrations set fish on edge, I would guess they might be a little stressed by music wiht ALOT of bass, or really loud music.
Does this make any sense?