It's all about Ohm's Law on the secondary of the output transformer. For the 300 watt amp:
The 4 ohm tap is wound to 35 Vrms --> 9 amps at speaker load 4 ohms;
The 8 ohm tap is wound 49 Vrms --> 6 amps at speaker load 8 ohms;
The 16 ohm tap is wound 69 Vrms --> 4 amps at speaker load 16 ohms.
All are 300 watts.
The 4 ohm tap is wound to 35 Vrms --> 9 amps at speaker load 4 ohms;
The 8 ohm tap is wound 49 Vrms --> 6 amps at speaker load 8 ohms;
The 16 ohm tap is wound 69 Vrms --> 4 amps at speaker load 16 ohms.
All are 300 watts.