If the manufacturer has correctly stated the nominal impedance in this instance 8 ohm, the 8 ohm tap should be used. For Wilson speakers all of which have a nominal impedance of 4 ohm the 4 ohm tap should be used. I own an XVX with a minimal impedance of 1.6. I use the 4 ohm tap with my 1.25 KW's. The amps never get more than warm and never require more than 120 watts.
Solid state amps should never be stressed. Folks that run their speaker with a 4 ohm nominal impedance off the 8 ohm tap will certainly get a "larger" amp that produces more current at the expense of over stressing the amp and shortening its life.
A typical example was the review of the Sasha DAW in Stereophile where the 8 ohm tap was used with the Mac 611 because it sounded better. I don't doubt this. However, the proper Mac amp for the DAW is the 1.25 KW which if used off the 4 ohm tap would sound better and significantly outperform the 611 used off the 8 ohm tap.
Here is the rule and I know this will result in a million comments: With Mac amps always choose the tap that best matches the nominal impedance. Disregard the minimal impedance. If the speaker is 6 ohm nominal impedance use the 8 ohm tap. If the speaker is 3 ohm nominal impedance use the 4 ohm tap.
If the speaker is less than 3 ohm nominal impedance and you really want it, use an amp other than McIntosh.
Best
Charles
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