Backbox and cab speakers are separately amplified channels, so your test with another set of speakers points to no audio output from the backbox channel on the sound board. Check J2 for cracked or open pins (pins 3 & 4 tied to ground, pins 1 and 2 tied back to pin 4 of U28 TDA2030A amplifier IC) Hopefully you get lucky and it is something simple like that. Could be a lot of things, but high possibility that U28 is simply failed - could have been overdriven, or something shorted speaker perhaps. Otherwise you'll likely need to audio trace the circuit with a oscilloscope.