new speakers 6ohm where do I connect to my MAC

A 6 Ohm load (according to McIntosh Techs) should be connected to the 8 Ohm taps.

My speakers are 6 ohms. i tried both 8 and 4 and preferred 4.

Where would you tap Martin Logan Speakers

I have an MC352 and from my experience with a 6 OHM load try both tap and use the tap that gives you the best sound. You will be able to hear the differet.

Your 2100 would probably be most happy on the 4 ohm tap. The older Macs are happiest within 80% of their tap, so 6.4 would be the minimum off the 8 ohm. The newer Macs have alot more current capability and can deal with about 20% off of their tap. The best advice has already been given, try them both, one will likely sound better. If not, use the 4 ohm so the amp will run cooler. Not all 6 ohm loads are the same. Good luck.