Well, since the secondary supplies in the SMPS are usually low-current, it could be that the "power out" you are observing is a thermal cut-out operating in the PSU. Again, it points to the PSU not really being the best choice for this; I would look at the ratings label on the unit and work out what current the 12V is meant to supply (continuous rating) and compare it to what the empeg and associated amp will draw. If it's greater than rated, the PSU will simply cut out to prevent overheating.

One of the few remaining Mk1 owners... #00015