Originally Posted by Slacktron
I was planning on purchasing an OEM sub with the old style harness for my car ('97 Legacy OBW)
I have an aftermarket head unit (Pioneer DEH-4000UB) that has a sub preamp out I would like to use. Is there any reason that I can't separate the +/- L/R wires before the [OEM sub's] harness and run them to the preamp outputs, while leaving the power and other wires connected in the T-Harness?
You could do that. However, the OEM sub is designed to run from speaker level (high level) signals. You may not be able to turn it up satisfactorily loud after switching to pre-amp signals. A long time back, someone posted about changing some of the resistors inside to convert the sub to run from line level, but he didn't post any specifics.
You may also need to add the remote turn-on lead (not included on the OEM T-harness), as the sub's built-in 'sound detect' circuit likely won't work with line-level signals.
On top of that, the OEM sub won't produce the deep lows you're looking for.
IMHO, it's not worth the effort. Look for a used aftermarket sub that already has line ins.