Tried another test with a portable music player, to rule out a source issue. Discovered something very strange, when fed a full-range signal, I could hear the lyrics coming out of the amp. I don't mean: out of the binding posts, along the wire and reproduced by the transceiver. No, coming out of the amp. I think something might be broken. Flipping the high-cut switch did stop the singing amp, though.
On the good news front, Guitammer contacted me, on a Saturday, to say that it seems that something is wrong with my amp. And that I should give them a call on Monday to initiate an exchange.
