4ohm/8ohm Switch?

[Sweetsound]

Hi guys,
My NAD 3150 amplifier has, like most NAD integrateds, a impedance switch at the back. Do I run the amp in 4ohm or 8ohm mode? And what is the difference sound-wise? My Celestion speakers are 8ohm...
Thanks

 

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[Sweetsound]

 

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[darrylo]

 

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[user 1447]

 

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