gm head unit integration

I want a preamp output from my 2011 GMC non bose headunit. Question: do any of the adaptors actually supply a true preamp output or, do they simply tap RCA jax off the speaker lines?

I have looked at the factory diagram and I don't see how an adaptor gets ahead of the built in amplifer. Hopefully I'm wrong and it is possible.

Yes, for now, I want to keep the factory HU. I hate the low priced aftermarkets even if they do sound better.

Adequate preamp voltage issue aside, are you are confirming my suspicion : a "line output converter" simply takes a speaker line and splices an RCA jac on to it? Maybe there is a resister pad in the splice or, better perhaps, a small transformer to reduce the current but none of them get between the preamp and the amp as those are strictly internal to the OEM head unit?

That would seem for sure on a GM "non bose" factory system where there is no outboard amp.

I wonder if the outboard amp in the GM "bose system" is also driven by the amplified speaker lines?