I want to know what is the best adapter for running an aftermarket amp and aftermarket door speakers with the facotry bose head unit?

I am using some cheap line converter and my strereo sounds nothing like it used to in my old Max with an aftermarket head unit. I am suffering a lot of distortion and engine noise!

If anyoune can comment about the Audio Link PD4 or the more common PAC Adapter, or recommend something different I would greatly apreciate it.