[Johk]

According to the 606 and 663 manuals, you should be ok.

But keep in mind that with lower impedance you will need more current from the receiver and therefore it can overheat and eventually shutdown (or break if the safety protection doesn't work) when you drive it hard (at high volume with all speakers on)

The 606 and 663 are made for 8 ohm speakers however they can handle 6 ohm speakers. Setting these receivers to 6 ohm (in their setup menu instead of the normal 8 ohm) will limit the power and therefore prevent overheating...

Since Onkyo receivers run fairly hot (from what I heard), I think the Yamaha may be a better choice for your application.