I have a Crate VTX200S combo amp. It has 2x12 speakers and pumps out 140W in combo mode. However, it can run a cab and it's wattage is bumped to 200W when it does. My question is does it disable the onboard speaks when an external cab is hooked up to it?
My gear...

Samick Les Paul (AV6VB) Guitar w/ SD Alnico2/JB
Marshall AVT150H halfstack
Schecter Stiletto Elite-4 Bass
Ampeg BA112 Bass Amp
It probably does not disable the speakers. My old Crate G130C runs a 412 plus the two internals. Just stick you ear up there and listen.
Just plug a cable into it even though you're not running it through and see if it cuts out. In my experience they do...but mine is busted.