This advice is a little late, but may help in the future. After making a similar mistake - forgetting how the wiring was connected; I now take a picture of the wiring as a reference; prior to disconnecting them.
I'm not clear what the problem is. Is the cabinet maintaining the temperature set on the Auber? If so then the Auber and heating element are working properly.
The Auber doesn't reduce output, it should cycle the element on and off to maintain the set temperature. I recall your goal was to run the Bradley at 300°F - by the way don't go beyond that The insulation in the Bradley will not withstand much higher heat. If you are testing the setup at 300°F, your 900w heater may have to stay at full power the entire time to maintain that temperature. Try your test at a lower temperature; such as 225°F, so see if the element cools off when it is cycles on/off.
If after testing at a lower temperature and the element doesn't cool down and remains at full power when the Auber is cycled off, then make sure you have your power cords properly connected.