[Arran]

What happened to the original speaker? Was it missing? If you still have it hold onto it, they can usually be reconed or rebuilt. I'm with Morzh on this, a proper filter choke is the better option for the power supply as opposed to a resistor. Though you can get away with using a resistor in a Pi filter arrangement in a 4 tuber like a model 84, and it will work fine if you use larger filter capacitors to compensate for the lower inductance, don't try it in a larger tube count set using 8 tubes or more. It's still preferable to use a filter choke (or a solid state substitute) as opposed to a resistor.
The reason is simple, a filter choke or field coil is an inductor, and inductors resist fluctuations (or changes) in voltage through inductive reactance, but they pass DC. A resistor resists a given amount of voltage depending on the current load, so the more current load there is the more voltage drop you will see across that resistor. Resistors also don't filter and regulate as well as chokes, so you will need larger caps to not only compensate for the voltage drop across the resistor but to filter out the extra ripple.
In the early AC sets high value capacitors were bulky and expensive, the only types they had were large value paper capacitors, so they used paper capacitors of 1-2 uf but used two or three filter chokes to get rid of the AC ripple. When they came out with wet, and later dry electrolytic capacitors, they suddenly had inexpensive high value capacitors in a compact package, so they could reduce the size and number of the filter chokes needed, usually incorporating the filter choke into the speaker field coil making it perform a double duty.
Regards
Arran