I don't remember iTunes, but there's apparently no way to UNCHECK all in Music.

The SELECT/DESELECT all options deal with coloring/un-coloring all items blue, NOT with the checkmarks.

The only way to uncheck all appears to be by hand...one at a time, but the order doesn't really jumble. What happens is that the unchecked items move down to the bottom of the list.

I suggest that you submit a feature request here.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire