I have a 2004 S60R with the HU-803 audio system, which sounds fantastic, but is lacking some of the modern features of the newer head units. My question is, if I upgrade to a new aftermarket head unit, is there any way that the OEM Volvo amp under the passenger seat can be used? I know companies like Crutchfield have the wiring harnesses to make a new head unit work, but I can't tell if they make use of the factory amp, as well as directing sound to all 11 factory speakers, including the center channel. I basically am wondering if there would be such a sacrifice in sound quality and volume to make the upgrade in features a step backward in acoustics. Any thoughts?