Pink noise or white noise is the right tone to use for this.
It is the playing of all frequencies simultaneously. It would be more difficult to discern the effects of comb filtering with a vocal. You need to use wide band content to really heard this effect clearly, like a full orchestra, but its still better with a noise color.
Keep in mind that demo is more severe than you would hear in a real MTM since that affects all frequency bands. That effect would especially be evident in the small wavelengths of tweeter bands, but that would not occur in an MTM center since it only has one tweeter.
By the way, you can do this same trick with an MTM center speaker by simply having someone rotate the speaker with a listener in a single location. That will let you know how much your center is affected by this phenomena. If you want to know what it looks like, see some of my center channel reviews. Look at this graph for instance, where the lobing causes patterns that make this graph look almost like a palm tree: