Less resistance means the amp can produce power more easily. Any decent amp will usually make more power into 6 ohms than into 8, but that isn't necessarily a good thing. As an amp nears the limits of it's power making ability distortion figures rise dramatically. It's one of the reasons I routinely suggest picking speakers based on efficiency--given 2 sets of speakers with similar sound characteristics being driven by the same electronics, the set with 3 dB higher efficiency will sound the same at half power as the set with 3 dB lower efficiency being driven at full power. That 3 dB difference will be evident as cleaner sound in the more efficient pair of speakers. Most often when speakers are burned up, it's not from too much clean power, it's from overdriving an amp into distortion.
Unfortunately, most speakers with high efficiency are low resistance, but sometimes the difference can be startling. Klipsch's "Klipschorn" speakers, huge corner horn loaded loudspeakers, were 4 ohm resistance but were extremely efficient--101 dB at 1 watt input. Considering most dynamic speakers run between 86 and 90 dB efficiency, that meant the Klipschorn's could play at deafening levels with very little power. Less efficient speakers would have been burned up by distortion as the amp tried to make enough power to play as loudly.
So, to more specifically address your question, although the Onkyo receiver is rated to drive 8 ohm speakers, it will drive 6 ohm speakers. It might even drive them louder, assuming both sets of speakers are of the same efficiency, but in doing so it might come closer to it's power limits and therefore sound more distorted, too.
It's a balancing act and a good rule of thumb is if it doesn't sound good to your ears, back off a bit. The human ear can easily perceive distortion levels of .25% and dynamic speakers have no trouble producing that level of distortion without much damage.
IMHO, that combo is a great pairing.
Doug
