If I understand your question, I believe the best you can do with the 125ESE is to use the white secondary tap (which would give you primary of 2500 at 8 ohms and 5000 at 16 ohms. With the single 16 ohm speaker it will be 100% mismatch, if you could add another 16 ohm speaker you could get it to 8 ohms. Fender has used 100% mismatch forever with the aux. speaker and it’s works (although not as efficiently as if it was properly matched). A better solution if you are only using one speaker would be to use an 8 ohm model.