[morzh]

Patric,

what you could do is this:

Measure your voltage with the 6100 Ohms resistor in place of the speaker. Then you will know your current for the speaker. Approximately it is 35mA or so.
Then you could possibly take any speaker that fits the size and has a field coil that is calculated to a similar current and use it with additional resistors to create the divider instead of the centertapped coil.

Or you could try to superimpose a divider on a speaker across the field coil, but then it has to be of much higher value than the coil itself and the current drawn bu the tubes' grids could act as additional resistance to GND, so you will have to try to adjust it to bring the voltages there where they want to be.