I did something similar a few years back in my Civic.
Used a 6 channel LOC to run the tweets and door speakers up front to an amp, left the rears on the head unit and then tapped into the "sub" channel and ran my subs to an amp.
Worked quite well.
2012 Mustang GT Premium - Lava Red Metallic - 6M - 3.73s - 401a - HID/Security - Comfort Pack - Brembos - GT Coupe Pack 5 - Continental ExtremeContact DW 255/40/19 - GT500 Mufflers
Ordered 5/17/2011 - Born 6/6/2011 - Delivered 6/23/2011