The output from the factory Head to the amp, is this line level or is it speaker level?


And what about the HK system

It's line level. The distribution of the power to the speakers is all done from the amp.

Well no, it's not quite line level, and it's not quite speaker level, its a biotch actually. It's closer to speaker level though. If you install an aftermarket radio, just wire the speaker leads to the input on the amp and you'll be fine (tried using the RCAs outputs and there wasn't enough juice to drive the inputs on the factory amp). Speaker level works, but volume is very sensative. The correct way to go is with a line level converter to reduce the voltage coming off of the speaker outputs on the head..