There has only ever been ONE car that had an OEM radio with RCA outputs, and it was a Porsche in the 90s. A good LOC will not affect the sound and it doesn't make it sound any worse than using RCAs, particularly on a sub, unless your OEM HU has some filtering on the channels you're trying to get signal from. You'd actually be better off running high level into your sub amp since any amp you pick will either have high level inputs (meaning there is a step-down transformer built-in which great simplifies the install) and the extra Voltage means you don't have to turn the gains up as much. Also, some amps' RCA inputs are capable of handling enough Voltage that you can simply add RCA ends onto your speaker wires and run those straight in instead of using an LOC or even using the built-in high-level inputs.
Again, it's a sub. There really isn't a wrong way to do it and it'll sound fine.