Ok, so my system is hooked up with a hi-low converter as it is now. Its actually hooked up to the rear left speaker in my car, then to the sub amplifier. I did not do this...the guys who installed the old system in my car did this. I'm putting in nice new equipment and I want it to sound excellent.
Does pulling the audio from that rear left speaker degrade quality? Are there particular brands of hi-low converters that are better than others?
Stock systems are a pain in the as$ to deal with. You said you want it to sound excellent, does that mean you're replacing the HU, speakers, and amplifier? Cause stock equipment is definitely not excellent.
If you just want to add subs and a sub amp, then you have to use a converter. As for quality difference, I would just look for one from a respected brand, not a no-name one from ebay.
in many cases a decent LOC can deliver a higher quality signal to the amp than a basic HU pre-amp output. LOC signals are high voltage, and balanced.
Pulling signal from that point in the system for subs shouldn't be a problem. Subwoofer material should be the same on both sides.
As far as the load seen by the HU. At the lowest, the impedance of the LOC will be around 100 ohms - depending on the type of LOC it could be MUCH higher -- effectively meaning the HU won't even notice it's there.