Integrated tube amplifier: 4 ohm or 8 ohm connection?

In a 4 ohm / 85db speakers using the 4 ohm bidding posts in the amp is obligatory? Or both can be tested, to hear which sounds the best?
Can the amp or the tubes be damaged?

amp: Cayin CS-55A
speakers: Duevel Planets
speaker cables: Atlas Equator 2.0

Thanks for your help :)