You don't really need a DC/DC adapter when you are in the car, since the Empeg was designed for in-car use. What I did was take a cigarette lighter adapter from Radio Shack, and splice on the correct size adapter for the Empeg's power connector. Make SURE that the negative/positive wires are connected correctly (use a multimeter to make sure). This way the Empeg recieves direct power from the car just like it would in the sled.
The 12V/2A adapter would work anyway, since the device connected to the power source draws the amperage, it isn't fed it. The Empeg won't draw more than 1A of power as per the spec, so you just need to make sure the adapter is rated at least to 1A.