Just as well I'm not paying Yamaha 50 squid a pop for these blown amps.
Repaired one end of 2011 , it bounced back.
It is due to the short stub lead between amp and its immediate speaker, the
elbow connectors.
No name elbows on lead marked UPOFC Classic Pro Loudspeaker Cable. Took a
bit of grinding to cut the top off the elbow, quite thick monkey metal. The
ground connection out to the cable solder point is like a solder tag but
with a 2mm x 1mm x tag thickness lug opposite the solder extension ,that
locates into the moulded plastic insulator. The tag is not spot
welded/bolted to the elbow metal so any twisting of the cable is transfered
to the tag and the ring part can twist and touch the conductor stem to the
tip, as the cylinder insulation along this stem does not extend into this
solder tag, or perhaps as only touching ground contact, then localised
heating and the cylinder insulation melting back a bit. I've not explored
that far in, the top moulded insulation plastic does not look melted.
These elbows have a vague brassy/coppery tinge of plating or something just
on the tips , the main elbow body is Hex in plan and between it and the stem
is what looks like a hank bush swage-form.