I'm just starting to upgrade my '11 GTs sound system, and I'm wondering about this. I do NOT want to ditch the factory SYNC nav system (and like you, I know the problem is NOT the head unit, but the line conversion), but I want to incrementally upgrade to all aftermarket speakers and amps. Do you have suggestions on aftermarket converters that will give me a GOOD preamp signal? I don't care about spending some money to make it sound good. After all, every speaker/amp will be high-end, and I'll be going for a SQ setup.
I remember reading about a product that JL Audio makes that is supposed to flatten out all the built-in factory bias, and provide a clean preamp signal. Just looked it up, it's called the CleanSweep. CL441dsp - Car Audio - OEM Interface - OEM Integration - JL Audio
Ever heard of it? I'm pretty sure that would give me a good starting point for a clean, adjustable output signal. I know JL's stuff is always top-notch, and the features sound impressive. What do you like? I'm stuck on JL for speakers and amps, but I would stray for something like this if it offers better performance.