Speakers with a lower impedance are more demanding on the amplifier, so it is possible you will need a higher quality amplifier. The specifications should give you some idea on how good the amplifier is. Seeing as there's the rule of thumb of 40 watts into 8 ohms at 0.5 % total harmonic distortion being enough for most home hi-fi requirements, a 90 watt amplifier into 8 ohms @0.5% THD should be more than sufficient. Specifications usually only give detailed information on 8 ohm speakers.
I haven't heard much about Pioneer speakers but, with the greatest of respect, there are brands of higher reputation than Pioneer who make speakers. You might find that a better pair of speakers would improve your set up more significantly than a new amplifier. Buying more efficient speakers (usually much bigger floorstanding units) will reduce the demand on your amplifier.
If you're setting up a surround system, the problem is complicated by the fact that surround receivers normally only tell you their power rating driving two speakers. A surround set up capable of 7 x 90 W, 8 ohm, 0.5%, all channels driven, would probably be quite expensive. I have read though that this sort of demand on the amplifier in normal usage is unrealistic.