4 8 or 16 ohms.

Always use the recommendations of complete strangers over your own hearing as hey, you don't want to insult anybody.

Thank you everyone for this explanation of ohms. I’ve been putting together sound systems since I was a teen but never bothered or had the need to understand this until now. One of my friends just gave me another example of this. I think it was a good one too so I will write what he wrote. Short answer try 4 and 8 ohms. The difference between 4 8 and 16 is how much electrical energy there is to drive all the speakers. 4 ohms is a heavier load than 16 ohms.  A 4 ohm speaker will extract more power from your amplifier than an 8 ohm speaker , about twice as much. Think of pushing a heavy ball up a hill. The load. The woofers take the heaviest load and take the most power to move. Most box speakers like B&W are 8 ohms settings overall but dip to 4 ohms. Other types of speakers like 16 ohms. That’s why the amp has different settings. If u connect just the two 800 bottom woofers it needs 8 ohms. But if u connect the top two woofers also it is twice the load so 4 ohms is needed. The bass should be strongest with 4 ohms. But u should also try 8 ohms. With 8 ohms the highs might sound clearer u will hear more detail but the bass will be leaner. If u put it on 16 ohms it will really sound lean. It’s all subjective to the music u like and what u prefer. So try 4 and 8 ohms. You need to understand with a tube amp it works harder with 4 ohms. The tubes will have the least life in hours. It is opposite to the Krell FPB 600 u had. With the Krell design 4 ohms is easier. But it still sucks a lot of power and 16 ohms is harder on a Krell amp. Just different designs. Your speakers actually dip to 2 ohms at some frequencies. The Krell was made for the 800s and provides a very punchy bass. It will be interesting to see how the ARC amp compares. It’s not about how much wattage but the quality of watts. You can drive the 800s with a 100 - 200 watt amp. Hope this helps. 

Tubes do not work harder driving 4Ω loads. The tubes drive the transformer  primary impedance which remains essentially the same. 

Transformers are happiest driving a matching impedance. When the impedance is mis-matched, so is the power. Tubes excell driving planars which have a mostly constant resistive impedance.

Roller coaster impedances like the B&W 800 Matrix are best served by transistor amps as they can supply the current necessary to generate the same power regardless of the load impedance. The B&W drop below 4Ω which can give some SS fits.

As far as sounding lean on 16Ω, the speakers are 16Ω at the very bottom, dropping to minimum in the upper bass and fairly constant ≈11Ω from 1k up. 16Ω could be just the ticket - depending on how the room affects the low end.

@tattooedtrackman Did you audition the AR amps before purchase?

@ieales  No I didn’t audition them never heard them live before but I read a lot about them. I’ve wanted them for years 

I think you should try 4 and 8 ohm taps and hear what sounds better to your ears.  There is no way that the 16 ohm tap would be the right ones to use as the B&W speakers are pretty well known to have lower difficult impedances.  As the other who have answered here said you won't hurt the amp or the speakers unless you turn it up real loud and get hard clipping.