When they do an
instrument amp like that, with a built-in speaker, they usually design the amp for the speaker they're planning on using. Hooking it to external speakers may be a problem - especially if you're planning two or more external speakers in parallel. The lower impedance will make the amp kick out more power... for a while. But while it's kicking more power to the speakers, it's also turning more power into heat inside its overworked transistors. It'll work, sort-of, for a while. But "a while" might not be long enough for the whole show. And for any venue larger than my living room, 100 watts
RMS isn't much to work with. I'd see if I couldn't find a rental outfit to cut me a deal on the use of a real amp in exchange for featured mention in the program notes or something.
John