A thing to keep in mind. In a "Series" circuit, the voltage drop at each "Load" adds up to the total volts available. So if Deve's numbers are good(I have no reason to doubt them) you will get something like the following.
High speed. 13.8 V. across the motor. That gives you 3.7 Amps to run the motor at 13.8 V.
When you add a 1 Ohm resistor(medium speed) you have a total of 4.7 Ohms. The Resistor(1 Ohm) and the motor(3.7 Ohm) you get 2.9 Amps to run the motor and drop 13.6 Volts across the motor. The other .5 volts is dropped at the Resistor. That Resistor now has .5 Amp to convert to heat.
With the other Resistor in place(slow speed) Resistor1(1Ohm) Resistor2(1 Ohm) and the motor(3.7 Ohm) you get 2.4 Amps to run the motor. And the Voltage drops to 8.8. With 3.2 Volts dropped across both Resistors, about 1.6 Amps across each one. The Amps is how you make "work"(speed) more Amps is more speed. In this case "work" equals fan speed. The Amps at each Resistor in slow speed is about 1.6 Amps that must become heat. The only issue that could come from a 6 Volt control at 12 Volts would be a High speed, almost High speed and a very fast low speed control.
The thing that happens in a 6 Volt system, is that 2 times the Amps are needed to do the same "work". So consider the following, 6 Volts and the same motor Resistance. High speed Amps is 1.9. The 6 Volt motor must have a lower Resistance or it would be slow and slower. With the same Amps(3.7) a 6 Volt motor would have to be close to 2 Ohms.
