Can I keep the factory amp w/new head unit
 

My apologies I'm sure the answer is here but I've yet to find it, 99 XK8 "premium" Harmon Kardon system, can I leave everything downstream of the headunit intact, and just hard wire the new unit to feed the factory amp? Do I have to replace the amp with aftermarket? And no i don't care about the changer, it's just ballast at this point........