Am I correct in thinking that you've connected 4v outputs from the docking sled, to the inputs of your amp?

Typically audio seperates used to output 1v signals, therefore amps were designed to take 1v feed. Then in the last decade some manufacturers starting upping the output of their seperates to 1.2v, to give the impression that their products sounded better, when all it did was sound louder. At the time this practise was frowned upon by the audiophile community, but I can understand why you might get a better fequency range, as source signal to the amplifier has been magnified by 4 times.

Hmmm, is it better for the head unit to do this amplification or the amplifier. I suppose it depends on how the empeg unit was designed. Are the outputs on the empeg seperate? Is this handled by the DAC?

This is just hypothesis, but I'm extrapolating from this that when your empeg is set to 0db, the volume is a lot lower on the amplifier than you would have on a another source (e.g. cd player)?