[FStephenMasek]

I replaced the 2,500 ohm field coil speaker in a little 4-tube TRF with a 1,500 ohm speaker in series with a 1,000 ohm resistor, and it works fine. There is a discussion of this on the Antique Radios Forum, and one guy there has a 450 ohm speaker on his bench for testing, and just uses different series resistors to match it to the particular set he is testing.

You are fortunate to have a 38-690.