Welcome, Mark!
To set multi-wave strength, choose Setup. You should see a MW strength setting above phase tune. Press the + and - to increase and decrease the setting. Six is the highest.
While low THD is in and of itself a good thing, the lowest displayed setting does not necessarily provide the best sound - although it may. 
As with everything else audio, it depends. Also, the built in metering if the PowerPlants while good, is not perfect. (Equipment which measures distortion is extremely expensive and, if built in, when more than double the cost of a PP.) Try the various settings and determine which sounds best for you.
Phase Tune adjusts the time relationship (phase) between the incoming voltage waveform and the regenerated waveform. With the Power Plant Premier and earlier P5 and P10, this was fixed. The fixed setting works quite well in most cases. However there are a few places (for example Long Island) that have unusually high distortion, and this distortion includes a very substantial time lag (phase shift) relative to the fundamental. In these situations, with the default setting, the Power Plant didn’t perform as well as it could. PS Audio worked closely with these customers to develop custom code that would help their situation.
99% of the time, the default setting (0) is appropriate and no adjustment is necessary. If your Power Plant is consistently delivering less than 0.5% distortion (THD) in Sine mode, you should not need to make any adjustments. Below are the conditions under which you might want to adjust this setting:
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Your power from the utility has extremely high distortion, over about 8%.
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Your power from the utility is typically more than 10V (15V for 230V models) away from your desired output setting.
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Your Power Plant is not able to reduce the incoming distortion by a factor of 10.
In other words, if your THD In reading is 8.0%, and your THD Out is 0.5%, your Power Plant is performing well. If your THD Out reading is 1.0% you may benefit from adjusting the Phase Tune. Please keep in mind that the Power Plant is correcting both THD and voltage errors. If your incoming voltage is within a few volts of the output setting, your Power Plant will be able to devote most of its capability to correcting the distortion. If the voltage is 10 volts or more away from the setting, the Power Plant is not going to be able to correct the THD as well. So if your utility voltage is more than 10V off, and you are seeing your THD reduced by only a factor of 5, this is probably normal.
The procedure for setting the Phase Tune is simple. Experiment with different Phase Tune settings and see what setting gives you the lowest output THD reading. The optimum setting depends on how much distortion is on your power and what it looks like. Unfortunately there is no way to predict what it will be.