I’d agree that the 6NS7 is preferable for preamplifier duty but in Kenny’s power amplifier the task/function is to be a "driver" tube for the 2A3 output tube. So as Aric described above the 6SL7 makes sense. Similar circumstances in my amplifier, the builder chose the 6EM7 over a 6SN7 due to the higher current capability to drive the 300b output tube.
Charles
Charles