Hi all,
running a new rogue audio atlas valve pwr amp, question is : my speakers are 6 ohm zingali's - 4 or 8 ohm taps?

Try both. I suspect the 4 ohm tap will sound better. How low does the speaker dip?

The two taps are intended to maximise power into the 8 & 4 ohm speaker loads but that is far from the only consideration.
You should also be aware that amplifier output impedance is lower on the 4 ohm tap and I would argue this is always preferable, assuming the amplifier can still swing enough volts on the output to give good volume.
High output impedance modifies speaker frequency response and that can only be bad IMO - it is a clear deviation from 'correct'.
There are part exceptions - speakers with completely flat impedance curves won't be a problem but try naming one. The closest you'll get are one or two full range drivers and even then we are are talking 'flatter' as apposed to flat.

So, your 4 ohm tap will better preserve the intended balance of the loudspeaker system and usually give you marginally better bass control.
Use it if you can get sufficient clean volume from the speakers.

i get loads of clear volume, more than i will use - sounds very good on 4 but wanted to check what i was doing is correct, Thanks Robert,
Piers

hi fi+ stated it didn't dip below 6ohm in their review

I suspect you will prefer the 4 Ohm tap, but always worth the experiment ;)