[JohnnyComeLately]

I'm pretty sure LOC is "Line Out Converter." It takes post-amp levels and drops it to a pre-amp level (usually between .5 and 2 volts). It's a cheap and easy way to add an aftermarket amp to an existing system.

If you go with an aftermarket head to aftermarket amp, you just go line level all the way. The tweeter jump is because you've got a separate line to the rear highs. I've wondered if I could just run a single set, and then split parallel at the rear speakers, putting in the cap I removed from the front door tweeters. I'm not highly motivated to do this as I'll probably just yank and replace OEM speakers with either Alpine 6X9s or 6.5" co-axial/components.