speaker level vs rca

When using a factory head unit is there any difference in SQ and power output of an amp between using a RCA converter near the head unit or running the speaker wires to the amp? Besides running 8 speaker leads vs. two sets of RCA's. Just wondering if the 50.00 for the converter is worth more than just the time savings involved. Also, due to price and size. I am using a pioneer 10" shallow sub. Would an 80hz crossover point be too low? Will be run with 6.5 3ways in the front stage, presumably set at HP 80hz. Can get a couple amps with that internal crossover cheap. Thanks.

There will be no audible difference.  The amp has probably the same exact circuitry in it that the line out converter has.  There are 2 reasons that I would use the converters.  First and foremost, the output chips in some newer factory radios cost upwards of 80 dollars.  That is if they are obtainable and you can even find someone to repair it if you were to drive a screw through the 48 feet of wire you ran through the vehicle.  A screw or a door sill cover, a seat bolt there are several ways that this can go wrong.  If you connect the converter to the wires behind the deck, the chances of this happening are drastically diminished.  Second reason, when you decide to change the radio out, you already have the cables ran.

80 Hz is a good crossover point to work with; crossing at 100Hz may work as well.  The sub's output may start to become directional, as when you can hear where the subwoofer is located, when you use the higher crossover settings.  Go with the 80Hz and tweak it from there.Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.