This amp seems to use a control line from the static ground contact of
the input socket, via a couple of 4.7K and 5V to pin 90 of the 100 pin
DSP chip. So normal operation is non-grounded of this static contact,
hence dummy plug required , at the guitar input, to use the external
signal input. I cannot see how that line could go LOW in guitar use and
inhibit the amp.