Your Creative speakers use RCA plugs and are designed for very low power levels. You'd need to cut the RCA plug off and strip the wire just to get it to fit in your Onkyo, but I don't recommend that. They will not sound good and you will probably damage them.
Now that we know what you have. You have a system that was powered by the subwoofer. Apparently that subwoofer died?
Anyway, the biggest issue will be what ohm load these are. More than likely they are 3ohm. That in and of itself isn't a big concern. 6 watts(more like these will handle 10) is much louder than you expect. But then, we don't know how inefficient these are.
Yes you can use them, but don't be surprised if one of them blows one day and all of a sudden sends the receiver into protection mode.
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