Xiao, as always, the absence of an "official" 4ohm rating shouldn't be considered a critical factor. To get such a rating under FTC amplifier regulations the receiver would have to operate into a 4ohm test load at the full rated power for at least five continuous minutes. This is a severe lab test which would never be duplicated in home use. Full power occurs only at brief peaks(if ever)and "4ohm" speakers such as the M80 are around that impedance for less than half their frequency range.

The 6006 should have no difficulties at safe(to your hearing)listening levels and no separate amplifier should be needed.

If you're able to buy from Accessories4less, they have excellent deals on factory refurbished Onkyo, Denon and Marantz units at that price or less.
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Enjoy the music, not the equipment.