Audio Systems: Factory sub replacement, rca inputs, pre amp

Question
My truck has a stock sub under the driver's seat. It is an 8 and has pretty impressive output for a stock set up but I want a little bit more. I already have a 12 in a truck box and an amp. But I like my factory radio and I don't want to rip it out. Can I take the sub inputs and run those to my amp in the speaker level inputs, without adding anything to it. I know that if you use regular speaker level inputs then you need to use converters, but since this is already a dedicated sub signal, can I do away with that?

Answer
I would have to guess since I cannot see the vehicle.
If it uses a pre-amp signal, you could easily run that signal to the RCA inputs of another amp and onto your 12", yes.
If it uses speaker level sends to the factory amp, then you could use speaker level inputs on your other amplifier. If your 12"/amp combo has variable input trim settings you might be able to adjust the inputs for speaker or line level(pre-amp). It really depends on the amp you intend to use.
I would not use any other crossovers on the signal if it is already set by the headunit or some other component in the system before the amp. If that factory signal IS full range, you will  notice it immediately when you turn it on through the new amp and speaker and will need a crossover for the sub to sound correct.

I hope this overview helps!

Sincerely,
Jerry Mael