Sonus Faber w/ Sonic Frontiers Power 1 - Impedance

I have a Sonic Frontiers Power 1 amp and Sonus Faber Grand Piano Home speakers. The nominal impedance of the speakers is 6 ohms. The amp has 4 and 8 ohm settings. Which do I use?
4 ohms! You can cause problems by having speakers hooked up to too high of a tap but you can always use a lower tap.
Thanks. Can you be more specific about the problems? Would I risk damage to speakers or the amp at 8 ohms? Or would the problems be in the realm of not achieving optimum performance from my system?

Usually, this would mean burning out your tubes. If you are driving the system hard and not paying attention, you could do more damage to the amp. Your speakers should be fine unless you are at 100% volume when somethings goes....
Honestly though, if you watch the tubes to make sure they aren't burning/glowing too hot, you can try the 8 ohm taps to hear the difference. The 8 ohm taps may get a very slightly warmer sound?
It's not worth the trouble though. Just use the 4 ohm taps and all should be great. I have amps with 1.4 ohm taps that don't care what they drive (2ohm, 4, 6, 8, 16) as long as it isn't less than 1.4.
I like your gear and don't feel that you are losing anything by using 4 ohms taps. That's what they are there for!
You generally want to find out the lowest impedence, not the nominal impedence, of your speakers, and set the impedence as close to that as possible.

With the SF amps, this means flipping the lid to move a wire, but it is easy to do.

Good luck.
4-Ohm taps and not too loud.
SF goes down to <3Ohms so you will eventually damage your tubes anywhay on higher volume levels.
Think of changing this amp for Manley(if you want to stick with tube gear) or add SS amp for bass(if you have bi-wired SF).
Tfk, as Raquel said above, I believe changing to 4 Ohms requires opening the chassis. I have a Power 1 and that's what the dealer told me the change involved. Is your Power 1 different? Does it have actual 4 and 8 Ohm taps on back?

fwiw--I haven't changed/rewired for 4 Ohms and am using on the standard (8 Ohm?) setting on nominal 4 Ohm speakers with no issues.
Hello, and thanks to everyone for responding. Unfortunately, and somewhat ridiculously, the POWER 1 amp requires de-soldering and soldering to change from the 8 ohm factory setting to 4 ohms. This pretty much takes experimentation out of the picture. I think I'll just make the change and see what happens. If I need to, I'll look into the Manley amp.
(With the POWER 2 & 3 amps, you don't need to solder. They have screw-down leads.)

Thanks again.