Beyer supplies a version of their DT-280 / DT-290 series headsets with a pre-amplifier built in to the headset, these versions are referred to as DT-280 v.11 and DT-290 v.11 series.  Apparently these versions look like a high impedance to the Tempest beltpack (Tempest BPs auto sense between dynamic and electret headsets by looking at the impedance of the headset microphone, the cut over happens at approximately 800 ohms).  This results in the beltpack switching to electret mode rendering the microphone output unusable due to the distortion induced.
The other issue that occurs on the auto headset function is a gain change on the beltpack microphone gain, this changes approximately 20 dB when the change over occurs.  These issues render the v11 series headsets incompatible with Tempest beltpacks. 
The normal version of these series headsets are compatible with Tempest beltpacks. 


















The wiring diagram for the DT-280 / DT-290 headset MkII compatible with Tempest beltpacks:






























The wiring diagram for the DT-280 / DT-290v11 headset MkII not compatible with Tempest beltpacks:





















The Beyer DT-280 / DT-290 headset data sheet is attached below.