HF, welcome. The general suggestion is to have the main speakers at a level where the tweeters are about the same level as your ears. With the M2s, this would put the bottom of the enclosure about 2 1/2' up.
The 5-6 feet from the floor recommendation that you mention most likely referred to the surrounds, not the mains. The surrounds play ambience and some effects which usually sound better coming from a bit above ear level, so a common suggestion is to have them mounted about 2-3' above the ears. This doesn't apply to the mains.
