Find Factory Head Clip at Line Level, 2012 GMC Terrain

My car, 2012 GMC Terrain, is pretty impossible to change the head on. I'm planning on upgrading speakers and factory sub, best way in these cars is to use line level. I'm planning the install around Audio Control ACM series amps. They have line level inputs and control remote turn on via those inputs. I was going to do the sub first. Factory sub is a terrible DVC Pioneer 8" in the rear compartment. I can get line levels from that. But was wondering if anybody knows if these are crossed over in a factory setup? I'd like to get my inputs from the same place, the sub is closest to were the amp location would be, opposite side of the sub behind the trim panel, but also want to test to see how high the volume on my head goes before clipping so I can get the best from my limited system. Can I grab the inputs from the sub, or is that crossed over in the factory setup? Sure doesn't sound like it. I Would be using an o-scope for testing. I've always tested from head back, but only done it on all aftermarket setups with RCA sources.