Until Apple fixes the dock entirely (by just allowing a few preferences hopefully), one method (there are a lot) is to add a “drawer” icon to each of your Stacks. The main icon itself will remain the same, but is seated in a drawer.
- Download one or both of the Dock Drawer Stack sets (say that 3 times fast)
- Open the zip file, extract the folder to somewhere (your downloads folder?)
- Open that folder, and then the DRAWERS_icon_#x folder. You’ll see a big list of folders.
- Find the (in this case Music) folder you want to apply to one of your Dock Stacks. Drag it to the appropriate Stack icon and let go.
- And now your Music stack has a cool drawer indicating that it is in fact a music stack.
via Optica Otima