On the switch: No see this vid for a good explanation,
I was suggesting an either/or scenario as the RC3 requires as closed switch condition whereas the Cube requires and open switch condition Boss FS-5/6 knockoff
On the External speaker connection
Option 1: use the recording/headphones Out Jack - look at the schematic and bypass the resistor network, of course you could just put a switch and switch the resister network in or out so you can switch between Headphones out and external speaker .... just mark it otherwise.....you'll be buying new headphones sometime
Option 2: mount an 1/4" female switch jack on the back of the amp
Just make sure if mounting it to the metal chassis that the speaker negative is the same polarity as the chassis otherwise use one of the plastic covered ones that will give you that insulation.... I seem to remember that its an 8 Ohm speaker but I stand to be corrected
