The DC-Block circuit blocks the small amount of DC from entering the power transformer. What is needed is a huge coupling capacitor, which is simply impractical, as 10,000uF/400V film capacitors might be made, but they would be bigger than a garbage can and cost a fortune. The workaround is to use two high-valued but low-voltage electrolytic capacitors in series with safety voltage clamps that prevent over voltages. In addition, the PCB includes a House-Ground circuit.
This is not a replacement for the existing PS-21, but is a brother tube power supply (note the B in 'PS-21B"). Just like the PS-21, the new PS-21B holds three independent regulated power supplies: one high-voltage and two low-voltage regulators. (Note the three heatsinks.) Unlike the PS-21, the PS-21B uses a voltage-doubler-rectifier circuit for the low-voltage regulators. Why? Answer: 6.3Vac secondaries.