Yeah, just leave off the RCA jacks. All 3 boards have same spacing for standoff mounts. Only issue is that these holes connect to ground, so metal standoffs can add ground loops. This configuration was the main reason I added a regulator to the PICCOLO, so that it could handle a +15V input supply.
Boards then just wire up together. They do not need to be stacked. Side by side is ok too. Or put PICCOLO on the front panel so you have easier access to controls (I has them on top).
jh