On the Genos speaker system (GNS-MS01) there is just one multicore cable which carries the line level signals from the keyboard to the amplifier in the sub unit, and also carries the amplified audio signals for the speakers from the sub unit to the keyboard. There is no MIDI signal connection! The OP (and the technician!
) is mistaken about the cable being for MIDI, although the plugs are vaguely similar!
Yamaha insist on using the same compact/cramped multi pin DIN socket as they used on Tyros models, but this time all the connections between the keyboard and amplifier/sub are made with this one socket and cable. On the Tyros4 and earlier systems the amplified signal for the left speaker was sent back to the keyboard via a separate 1/4" audio cable.
The multicore cable was known to give problems on Tyros models if it was flexed a lot, but this didn't usually cause the amplifier board to fail! I assume that Yamaha expected that the cable would not be connected/disconnected or flexed a lot since the system is intended for home use.
However presumably on the Genos version if a short circuit or open circuit does occur between pins or conductors in the cable, then unless the amplifier's speaker outputs are protected against short circuit or open circuit, this could cause the amplifier board to fail.
They may already incorporate such protection in, but if not then Yamaha need to redesign the amplifier board with such protection so that problems with the cable or sockets do not cause a meltdown of the main amplifier board!
Regards
Ian