I've been playing with the IBus and reading the messages on it. This has thrown a little more light onto how the DSP and the radio interact. I can confirm that the radio does send "volume increase" and "volume decrease" messages to the DSP. It also sends the Bass, Treble, Balance, Fade, DSP and Subwoofer settings to the DSP. So it would appear that the volume, Bass, Treble and Balance controls are implemented by both the head unit and the DSP, whereas the Fade, Subwoofer (and DSP) controls are only implemented the DSP. Of course the Sub and Fade controls do affect the relevant outputs from the radio but these outputs are not used by the DSP. This is quite curious behaviour but I think it is down to the radio and the DSP being made by two different companies and Alpine wanting to use the head unit design on systems that don't have a DSP.