Newer Mac amps have more current reserve than the older pre quad balanced and current analog designs so they more than meet specs driving a 4 ohm load from the 8 ohm tap. That said a lot of speaker manufacturers today are pretty negligent with their impedance ratings. So if the speaker reaches even lower and might require the use of the 4 ohm tap. Without an impedance bridge you are going to need to depend on your ears and Power Guard lamps. If the amp starts flashing prematurely on the 8 ohm tap as show on the out put meters then the 4 ohm tap should be used. Or if the amp get prematurely hot and goes into protect. According to a test by Stereo phile the speaker dips below 4 ohms from 70 to 300 hz with close to a 2 ohm reading. So I would use the 4 ohm tap. 70 to 300 is where most of the fundamentals of alto, 2nd tenor, baritone and bass voices and musical instruments fundamentals lie.
