4 or 8 ohm tap - is this a good way to tell

The 8 ohm tap produces a higher voltage (for a given input signal level), and will therefore sound louder. Loudness does not imply a better match. However, this higher voltage comes at the cost of reduced current capability, so the maximum power capability will be less if the load is not matched to the amp's output.

If the speakers are 6 ohms average I guess it makes sense to use either 4 or 8. As Shadorne suggests, pick the one you like, and don't worry about it.
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Whatever sounds best to you. Either way the impedance of the output transformer stage of the amp will be modifying the frequency response of your speaker in accordance with the impedance variation of the speaker with frequency. If you have some test data of your speaker impedance response with frequency you can have an idea of what sort of equalization will be achieved. ( Wherever speaker impedance is low you will get a reduced output and wherever the speaker impedance is high then you will get an accentuated output.) Since you are not seeking a flat response then it is best to go with whatever sounds best to your ears.