How do you tune a line converter?

I saw some high-low level converters that had little gain switches for each L and R channels. How would you tune those? Also, there were some that didn't have those dials, are those better or whats the best thing to do?

(I've always used the sub outputs on head units but my friend wants to keep original head unitHow do you tune a line converter?
the best thing you can do is get the right head unit to do the job the right way. if its a must i would buy the best line out converter and they must come with instructions to set it.or if the best has no settings then you best read up on it.tell your friend a new head unit that is made for running subs and speaker amps is the best way to go if you want a powered system,4 rca ins for a 4 channel amp and 2 sub outs to run your subwoofer amp, and have some control over how to mix your speaker amp with a mono amp that runs sub woofers.the head unit is like the heart in your body to a powered system.if you go with the stock radio and a line out converter,you are never going to get rid of the static and or feedback in your speakers from the half azzed RCA cables running the amps.do it right once or do it half azzed and it will piss you off every time you turn on the stock radio,not made for running powered system.talk to your friend and smarten him up.good luck with whatever you do.