2ohms, 4ohms, 8ohms, which to hookup too?

If I understand your question correctly you are only connecting one pair of speakers. If so, either the 4 or 8 ohm outs could sound best depending on the actual impedence curve of the speakers. In my experience, with effecient speakers where the amount of power available is not a serious issue (you've got more than enuf for loud listening) I have preferred the 8 ohm taps of my amps. You should use your ears to determine which taps to use.

If you are talking about connecting both pairs of speakers (I can't imagine why you would) you would probably want to use the 4, or more likely the 2 ohm taps.

All good advice so far. I would add that while one might be theoretically correct, sonically you might prefer another.

According to McIntosh Lab, when in doubt, hook up to the lowest Ohm. It shouldn't matter much, since the MC300 is an autoformer, which means it will deliver 300W/Ch. to whatever load.
Good luck,
tim