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.
