I would cross over at 60-80 Hz to the sub. This will let it do it's down and dirty work, and then your receiver will not have to work so hard powering your mains. This is also personal preference.
Two identical Klipsch subs are going to put you in the money range for some nice stuff from SVS, HSU Research, Epik, eD, etc. If you want two pretty subs that look like your mains, center and surrounds, go for it, buy two Klipsch.
But... Better performance can be had for the same money...Your call.
Read up on this bad boy, the flagship for SVS