I've lost track of how many times my MXR M80 has saved me from situations like these.
Most amps will have at least a workable power section and any speaker is better than none. Run the preamp into the effects return of the amp and have DI straight to the board so at least the audience get a good sound. Actually, the MXR and pedals like it are just generally useful to have. Last weekend we used it as 'flat' DI for the acoustic guitar in the opening act, then turned it on and used the unaffected out for the first band's rig, then the effected out for my rig, didn't have to unplug a cable once. Sooooooooooo many uses for a tone shaper, DI, output split, distortion pedal . . . . . . .