If you are just going to use one pair of 6 ohm speakers there is no problem!
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The Amp site is stating the power at different impedances which helps show how stiff an amp it is. Ideally it doubles power as impedance halves.
The back is telling you the range of impedance that is safe to use. If you use 1 pair of speakers, you may use 4 Ohms or higher.
If you use 2 pairs at once, you must use 8 Ohm or higher speakers.