[Radioroslyn]

Sound like a plan. The 125E should a good match for either an original or pm spkr. As a practical matter the resistance of the field coil is not very critical as it is in the positive leg of the p/s. Some sets the fc is connected to the HV ct and the negative bias voltage is developed across it. Theses are more critical it will effect the bias voltage.

Your set has what Philco calls there 10w high fidelity amplifier, it's p/p 42's wired in a triode configuration (plate and screen tied together) And a single 42 driver tube.Set up like this it delivers a good low distortion output. If in were in the standard pentode set up it would have more gain (would take less audio to drive it) but more distortion. Great for listening to some snappy tunes from Paul Whiteman's band!
Another thing to have a look at is the driver transformer. Sometimes they can have an open pri or sec.

A few of the big Philco sets have the 15w high fidelity amplifier which have larger triode tubes like 6A3's or 6B4's and a 14" spkr
https://www.youtube.com/watch?v=1w1P1VK9jGs