AC15 is Push Pull, based on the type of output transformer. If you look at the schematic, the high voltage comes in at the centre tap and out to the power tube plates
via the taps at each end of the primary OT winding.
A single ended transformer would have the high voltage coming in at one end of the primary and out to the tube (usually one, but can be more) at the other end. There is no centre tap.
I doubt very much that the AC15 power tranny is rated at 400-0-400 at the B+ secondaries, this would equate to a plate voltage in the region of over 500 volts, maybe 400VAC end to end?