Just an update: had the amp, cap, and sub installed a few weeks ago. The tech tried first with an LOC tapped into the rear speakers after the factory amp. It sounded pretty good but was getting some signal interference from somewhere with a constant soft "beat" that was noticeable at low volume levels, almost like it was trying to ground itself out with the factory amp. He then moved the LOC to tie into the wires pre-factory amp as ttredraider78 suggested. Now it sounds so much better, both stronger and cleaner, with no signal problems or interference at all. Couldn't be happier with the amp and sub! Thanks again for the advice!