6ohm speakers to 4/8/16 ohm amp?

Let your ears be your guide; you won't hurt anything by trying the different taps.

In my personal experience, my Andra IIs are also nominal 6 ohms, and I have had the best results using 4 ohm taps on my Dodd tube amps.

Thanks. So forgive my ignorancne but what is nominal impedence and what is an impedence curve?

Depends on the actual minimum impedence, not the nominal impedence, and the impedence curve. That said, you should listen to the speakers connected to the 4 ohm taps and the 8 ohm taps and pick the ones that sound best to you. Typically the 4 ohm taps will produce better bass frequencies IF you need to use them.