[elerner61]

The following applies to the Top HiFi with NAV, DSP and Carver subs system. I do not know if this is true for the other audio options.

Volume control take place in the factory amplifier. The factory head unit digitally communicates (via the KBus as I understand it) to the amplifier and tells it to increase or decrease the amplifier gain. If a factory head unit is not connected to the amplifier, you will not be able to adjust the amplifier volume (gain).

If you could just set the amplifier volume to full, leave it there and then connect a non-factory that might seem to be a solution. But you can't do this. The factory amplifier volume is reset every time you turn off the car. Try this, play a tune at a loud volume and turn off the car. Now restart the car...volume is reset to a lower setting.

If you are going through the hassle of replacing the head unit, then go the extra mile and replace the amp and speakers. In the end you will get an immensely better sounding audio experience.