In my 99 gs4, I want to hook a LOC to the factory
sub. Then to a Rockford amp. My question is, since the factory sub output is probably lowpassed, do I still need to set my rockford amp to lowpass? Also is there such a thing as too much lowpass filtering?
Thanks

 

I would still set it to Low pass, there is no need to amplify the high end of the audio signal and then try to make the factory LPF (low pass filter) filter out the probably-too-amplified high frequencies. There is of course too much of anything, in any filter there is loss and noise, but in this case you only have two in series, and I would leave it to low-pass if it were me. If the filter is in the amp and not the sub wiring, and you are replacing the amp with the aftermarket one, then you should definitely use the Low pass filter on your amp. If you are planning on using the output of the Lexus amp to the input of your Aftermarket amp I would hope you have speaker-level input, sometimes you can get away with tapping into a speaker to an RCA input, but only if it's a very weak system to begin with, and I don't think that would be the case for the output from the lexus amp.

Sorry, just noticed the LOC comment in your post, so ignore half of what's above, but still note that the LOC is for speaker level, and not necessarily Sub level, which is much higher.

So in short: Yes, use the low pass filter.