You have to be careful, in that even "6 ohm" rated speakers can have their impedance drop to values a lot lower than that depending on the freq.
If you do wire in series, typically you don't have to change levels much, because you're now delivering the same total power to 2 speakers anyway. (Each speaker gets 1/2 the power, but since there's 2 of them, you end up with roughly the same volume.)
But it is a good point, that surrounds do carry a lot less content than say the front speakers. I don't know about the CSW's, but Def Techs are fairly high sensitivity (efficient) so that helps you if you do go parallel.
Is your receiver still in warranty?
