[Arran]

 Those capacitors, C35 and C36, are between the plate of each 6K6 power output tube and ground. The maximum power output of the amplifier is only rated at 5.5 Watts with 220 volts on the plates so they are not exactly overdriving the 6K6s to start with so maybe they used 1600 Volt caps because they wanted .0035 uf caps and got a deal on some 1.6 KV ones, or already had them in stock and didn't want to order lower voltage ones. If those two capacitors were connected between the center tap of the output transformer and the two plates, or were connected between one 6K6 plate to the other 6K6 plate across the transformer primary then I would say that 1600 VDC caps would be needed but I think that you might be able to get away with using a pair of 630 Volt rated caps in this case. 
Regards
Arran