[Arran]

It looks like what you have in this set is a combination power switch, and tone control switch, the capacitor in question appears to connect between the plate of the 6SQ7 tube and ground, basically it is switched in and out of circuit to cut the trebble. If it is an original part, and not a replacement, I have no explanation as to why a 1600 volt cap would be necessary, the highest potential that cap will see in that position is maybe 80 vdc. If it were a tone control with a potentiometer, and one end of it were connected to near full B+, and the wiper was connected through a cap to ground, then I could see a reason for a 1600 volt cap in that position since if it shorts out or becomes very leaky it can burn up the pot.
Regards
Arran