Pakwong, using zeros will not help, and could possibly hurt. Tube amps like the ARC Ref 150 work best if the speakers impedance is perfectly flat as a function of frequency response.
Take a look at John Atkinson's bench test results of the Ref 150 and look at his measurements of output power and distortion as a function of speaker impedance. The amp's ability to produce clean power is attenuated if the load impedance deviates from the amp's nominal impedance at the relevant output tap.
This is not a big issue because as Al (Almarg) recently remarked, a rough rule of thumb is that an amp's power output demand is roughly 50/50 at the 300 Hz frequency. Stated differently, one would do better to match the speaker impedance in the low bass/midrange frequencies to the amp's nominal impedance at the relevant output tap.
An example might help. If your Maggies present a flat 4 ohm impedance load as a function of frequency response, then I surmise that the Ref 150 will deliver close to its rated 150 wpc channel with the least distortion ... if the 4 ohm taps are used. By contrast, if the Maggies impedance is between 3 and 5 ohms between 20 Hz and 500 Hz power region, then rises to 8 ohms thereafter, I surmise that the Ref will still do a pretty good job delivering clean power into that load if the 4 ohm taps are used.
OTOH, if the Maggies impedance is between 3 and 5 ohms between 20 Hz and 500 Hz, then rises to 8 and 15 ohms thereafter, I surmise that the Ref ability to deliver clean power may be a tad attenuated if the 8 ohm taps are used. The reason is the amp is being asked to drive a low impedance load in the power range when speaker and amp impedance is not well matched.
Hope this helps. In the end, most tubies like me say, go with what sounds best. Nevertheless, I am a little concerned as to whether the Ref 150 can bring out the best in the Maggies.
