[dmblackburn]

I found a K-17 speaker on eBay that is very similar to the K-7 speaker that is native to the Philco 71B, but wondering if I can directly substitute it. 

K-7 specifications: Impedance: 1.25 ohms, output trans primary: 450 ohms DC; Field coil 1125 ohms DC

K-17 specifications: Impedance 1.25 ohms, output trans primary 310 ohms DC; Field coil 1125 ohms DC

[morzh]

See the tubes they were used with.

K-7 seems to be used with 42.

then you simply compare the tubes' output inpedances for the same output type (provided they were both single ended, or both pushpull) and see if there is a difference.
You should not use push-pull transformer for SE and vice versa.