Electrical – the base current of a class ab amplifier with darlington pair configuration

I think you are picking up AM radio interference because of the relatively high resistances you are using. High resistances make it easier for a low energy signal like a radio wave to imnpress a voltage on a circuit. I would suggest 2 remedies. One is to reduce all of your resistors by a factor of 10. Why do you need to put such a large resistor (R4) in series with your output? Another remedy is to put a small bypass capacitor across R2 to filter out the AM signal. If you reduce R2 to 10k, then a capacitor of about 200 pf should be sufficient. If you reduce R2 to 10k, then use a capacitor of about 2000 pf.

I am not really an audio guy. But I took a look at Douglas Self's "Audio Power Amplifier Design Handbook." Those values, of 55 to 59 mV are ballpark. You might want to warm up the amp (by playing some music) and then check it again when it is warm.

As far as the mismatch goes, it isn't a huge mismatch. But were you measuring it with a load attached or no load? With no load, the two resistors are perfectly in series (or so I assume), so the currents have to be the same. In that case, any voltage mismatch is due to Re mismatch. You can tune Re by adding resistors in parallel with whichever one is higher in voltage until the two voltages match each other. Again, do this with no load attached to the amplifier.

If the two Re's are already matched, but you have different voltages across them when a load is attached, that is due to an output voltage offset. Unless it is large, I wouldn't worry about it.

Ultimately, bias current should be set based on what produces minimum distortion, not on anything else. But it is not very practical for most people to measure distortion while tuning bias current.

Good luck!