The ARC scenario you describe is very different from that in the Bogen amplifier, and so requires the use of a much lower reflected impedance from the output transformer.
In the MO-200A setting, the screen voltage is at best half that applied to the plates, which itself is greatly elevated over the voltage supplied in the ARC design mentioned. In Bogen scenario, the required plate-to-plate load for any given pair of tubes is increased to the highest end of the range within which the tubes can work in conventional pentode circuits.
In the ARC scenario, the screen and plate voltages are nearly equal in value, with the screens operating at a higher voltage and the plates at a much lower voltage than in the Bogen. This scenario is very different then, requiring a much lower plate-to-plate load impedance for any given pair of tubes, which now appears at the lowest end of the range within which the tubes can work in conventional pentode circuits.
If the required load impedance of one scenario is used with the operating conditions of the other scenario, then neither end result will produce near the power output that the proper pairing will produces, distortion will be greatly elevated in both, and in the case of using the load impedance of the ARC amplifier with the Bogen operating conditions, will result in greatly over dissipating the tubes as power output is increased.
It is always desirable to use the lowest possible OPT primary impedance, as doing so greatly alleviates the design issues associated with an OPT's HF frequency, power, and transient response performance, particularly as the physical size of the transformer is increased to handle greater power output capability. Winding capacitance can become a serious problem at higher impedance levels if not carefully accounted for in the build of the transformer.
Dave
