Is there a capacitor strapped across the mains switch, or the input
socket? That would account for the clout he got when touching the mains
plug pins

Ron(UK)

There is a .47uF , 330 V polycarbonate between neutral and live on the amp
side of the mains switch which would self discharge through the transformer
in mS but nothing else except a fuse.

Are you sure it`s on the transformer side of the switch?

Ron