For simplicity and tonal response I would look to the Audio Control 6XS. While it is not advertised as a LOC, it is indeed a advanced LOC. It will afford you a crossover at a higher slope than most amps, a master input gain control, output gain control, a built in subwoofer level control with volume knob that mounts up on the dash and of course, the LOC. All you need to do is take your factory speaker wires, extend them to where the 6XS will be mounted, solder on a RCA end to each one and that is it, done. This works great if the factory head does not have a heavily equalized output or is not in a multi channel output (being more than front / rear / left / right).
For more advanced again, as in 8 channel output, aux in functions, the LC8 is the way to go.