It won't damage the subs or the receiver at all...unless they're DVC or have multiple SVC drivers, you can't "choose" what impedance you're going to have.
Is it a preout or what?? Or is the reciever POWERING your sub??//content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif
Most receivers have subwoofer preouts and NOT speaker outputs.