I had a 15v/450ma power supply that didn't supply enough power to the Empeg and it kept trying to boot, but then it would shut down its display.

The empeg-car label on the top of your unit has a nice little picture showing the polarity of the connector and 11-16V DC 1A. Your power supply is only providing 450ma which (and remember I'm a software bod so this could be completely wrong) is probably causing the voltage to dip below 11V thus causing the powerfail to kick in.

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Mike Crowe
I may not be speaking on behalf of empeg above :-)