6 ohm speakers with 4/8 ohm amplifier

8 ohm may be fuller 4 ohm may be a little lean

try both it's fun!

I think the correct answer depends on the impedance curve of the speaker. 6 ohms is simply the nominal impedance. Without knowing the particulars of your speaker, I would say that the 4 ohm taps are the better choice, since the impedance will be lower than 8 ohms for much of the time.
As was said before, try both. It's possible you won't hear any difference. Only if the speaker's impedance drops into the 2 or 3 ohm range for a significant part of the audioband will the 4 ohm tap sound noticeably better.

For what it's worth I have revel performa30 with an arc vsi60 , base it tighter , Mids are cleaner on the 4 ohm tap. I wasn't happy with amp until I changed from 8 to 4 ohm

I have 6 ohm nominal KEF Q300 speakers and I use the 4 Ohm taps of my Jolida JD-502P amp. I found the bass to be slightly louder and tighter vs. the 8 ohm tap of the Jolida. I figure the KEF Q300s may dip below 6 Ohms so I might as well play it safe and use the 4 Ohm tap just in case.

Say your speakers are 6 ohms and your amp gives out 120wpc in 6 ohms.
So, if I am shopping for an amp for my 6 ohms speakers and care about the amp’s power rating, I should go by it is power rating at 6 ohms, correct?

I should also mention that I am just asking a technical question, not what I need/want as power or not.