Normally, I would advise people to wait until actually having the speakers and trying them with the receiver to see if it is powerful enough or not before blowing money on an amplifier that may not help anything. In your case, according to the manual, the receiver isn't supposed to be used with impedances lower than 8 ohms, so that may be a concern in this case. On the other hand, the speakers are supposed to be pretty efficient, so it may not be a problem. You might want to try to find the minimum impedances for the speakers (the nominal impedances for the three front speakers are 4 ohms), and try to find reviews of your receiver in which a reviewer tried an impedance lower than 8 ohms. I would also contact the speaker manufacturer and the receiver manufacturer to see what they have to say about this matter.
If I were to hazard a guess, I would imagine that you could get away without any extra amplifier at all, if you listen at sane volumes instead of loud enough to cause permanent damage to your hearing, but that is a guess. If one over-stresses an amplifier with too low of an impedance, it can destroy the amplifier, so it is good if one is not guessing too much about such things. Hence, my suggestion of trying to find appropriate reviews and contacting the manufacturers about this.
