[morzh]

It cannot be told off hand, you need to know

- what current the speaker's field coil is designed for
- what ACTIVE resistance you are adding in series (either choke's DC resistance or resistor, or oth if you add both)
- field's own DC resistance
- consumption of the radio.

Now, for the most part, from my understanding of tubes, average current consumption does not strongly depend on voltage so adding active resistance will not affect it as much (to a point of course). So, if you stick a field coil that was designed to work with 50mA into a radio that work with 10mA, it will be weak, and vice versa, it will be too much and no amount of added resistance will help.

So I would look for a speaker that was designed for a similar radio, and if a resistance is less I would add some to compensate.

Like I said before, it will be a search for a compromise.

In practice though most speakers will work in wider range of parameters scattered around of what they were designed for and you have to look to not bring it into some extreme incompatibility mode, where it will be too weak or too overloaded, otherwise it will probably be fine.

And an extreme case is easy to see.
In other words, best is enemy of good enough