4 ohms , 6 ohms, or 8 ohms???

The thing about a set of ZEROs is that they can allow you more flexibility.

For example you could run the amp straight into the mid/tweeter section while using the ZEROs for the bass section (assuming that the speaker is bi-wirable).

OTOH you're not going to hurt the amp trying different taps so I would do so. But it does seem as if the 4ohm tap is likely the most appropriate. At any rate, keep your speaker cables short (under 3 feet is a good idea) and make sure the speaker connections are tight.

I don't know if this applies to all amplifiers or not but my speakers - Martin Logan Aerious i, sound noticeably different depending on which taps I use - 1, 2, 4, or 8, on my McIntosh MC2200 amp. I attributed this to the  speaker cable length I'm using and the variations in the damping factors and the associated output impedance for each tap as described in the owners manual.
8 ohm - DF 16
4 ohm - DF 30
2 ohm - DF 50
1 ohm - DF 16
For my 25' speaker cable run and 4 ohm speaker impedance (minimum 1.7 ohm @ 20KHz), I've found the 4 ohm tap sound the best with the 2 ohm follow real closely with the most impact on the size of the soundstage. With the 8 ohm tap the soundstage becomes very diffused.