If Ur amp has a possibility of bridging chanels, than it probablly has also a cross inside. Did U check it out - usually is a switch with a sign something like HP- LP filter.
@ Dan - Your amp will have a crossover (or filter) built in.
As Tom has already asked, if you are running from a sub pre-out on your head-unit, your head-unit will take care of the filtering (and will be adjustable from it). If not check your amp is set on low pass.