Not got inside yet and before doing so, less I disturb something.
Inserting a mono jack in one of the input channel I/O Insert sockets,
kills throughput to phones and also amp hiss at the phones output. The
PMP1280 SM ,out there, does not seem to have a control line attached to
the I/O insert socket switches at all , so different there , unless
there is some sort of auto lack-of-signal mute circuit somewhere. At the
moment I wanted to rule out something silly in the way of a wrong switch
setting combination or something. The user manual shows nothing that
could cause this.