The music should be free or it shouldn't. There shouldn't be a gray area.
The faster any laws about this get resolved, the better. That's why I want this to be public.
There is no question that RIAA is no angel, I'm just saying you're complicating it for muxtape and the direct result might be them not being able to provide an awesome service or them having to make it much less attractive.
If this is public, muxtape potentially shuts down; RIAA does not change its unethical practices. Think about it.
Perhaps a better way of going about this would be to contact the muxtape developers directly, however like you said before, they already know about this.
Also, in this situation, the RIAA would not necessarily be doing anything unethical to shut down this site. They are hosting free music on the internet. Can you think of a more straightforward way to break copyright law?