If you look at the Class A S/E (single ended) and move over to the Zout, you will see the impedance for the 6K6 is 9000, and the 6V6 is 8500 where the 6L6 is 4200. You would want to run a 6K6 at the 6V6 position.
I’m not sure what you mean by what tubes were used, do you mean brand? If so, I don’t know what the brands were.
400v is a workable voltage (and probably sounds great), the 40uf (or even a 30uf or less) cap should not create a hum issue in conjunction with the neg. feedback loop. I would double check the layout and lead dress.
I realize it should not cause a problem but it does.
I used a point to point bus grounded layout with no board but there is nothing around the feedback resistor from the output switch. All the AC stuff is at the other end of the amp. The amp is dead quiet without the feedback resistor but hums with feedback connected.