Everything else being equal, a lower impedance in a speaker will lead to a higher current draw from the receiver. I don't know it that equates to a higher power draw, though. The higher current draw is why low-impedance speakers (like, 2 or 4 Ohms nominal) are a challenge for many receivers. 6 Ohms won't be an issue at all for that Pioneer, btw.
Those EMP speakers aren't extremely efficient, so they'll need more power to play at equivalent sound levels compared to more efficient speakers. The efficiency (or sensitivity) on those is 87dB. Now, that does not mean that the VSX-21 can't play those loud - not at all. It might mean that the VSX-21 won't be able to play them at really loud levels while still maintaining all the dynamics of the music. I noticed that will my Pioneer VSX-1015 and NHT speakers. At normal to loud listening levels, everything sounded great. At once-or-twice-a-year levels (LOUD), it sounded a bit muffled (best word that I can think of). Sounded okay, but not quite right. When I added an external amp for the front speakers to lighten the load on the receiver, it sounded fantastic even at those louder-than-normal levels.
In summary - that Pioneer likely has plenty of power for you unless you really like to crank it.