It seems to me that the challenge for the "objective" theory of aesthetics is pretty much the same as the one for moral realism. Yes, the fact that people disagree about artistic value doesn't mean that artistic value is subjective - but I'm yet to be convinced that it's objective in any meaningful sense. In the end, though, you're right about disagreement over political systems (despite some attempts to do away with political relativism, e.g. Badiou) - but only because different people have different, mutually exclusive, visions of what society is and should be, and the methods to get there. The arguments you're talking about in politics only have any meaning once at least one set goal is agreed upon - e.g. human freedom. But I struggle to think of any such goal that the Bach scholar and the Harry Potter fan are going to agree on.
I think there's a reason why even die-hard Adorno "fans" have given up trying to argue why jazz music is so bad.
When someone can think up an argument to convince a Harry Potter fan that they should really judge Bach's music as more beautiful, pleasing, of higher artistic value etc. than Harry Potter, then I'm curious.