You could easily make a speaker out by clipping the speaker wires from the amp section to the speaker, in between; and attaching a male quarter inch jack to the speaker side and a female quarter inch jack to the side going to the speaker from the amp section. Plug these in to use the built in speaker.

Unplug the quarter inch jacks and run a speaker cable to a properly matched ohm speaker cabinet from the female jack coming out of the amp section.

Wouldn't this make for a versitile way to use your amp as a head for a properly matched cabinet?

I have always considered doing this to a couple amps but haven't gotten to it.