hey Bridgman, you're right, our receiver do not have any built in xover network, you might want to check out this xover site: http://www.the12volt.com/caraudio/cross.asp

I do not know how good were the xover from those subs, but the 1st order setup is pretty much just a capacitor so i'll look for some high quality stuff. I' might cross them at 90hz to roll off a bit sooner.