Our church recently bought a Shure SM2 headset for the keyboard
player.  Balanced monaural audio is sent from our mixing board down
the snake to the phones in the SM2.  The left and right dynamic
headphones are each connected to audio transformers within the SM2. 
The primary windings of the transformers are tied together on one
side, with the other side of each primary brought out separately for
connection to the audio source.  What is the best way to connect the
balanced monaural audio from the board, to the SM2's headphones? 
Should we leave it balanced and connect one headphone to the balance
monaural hot and the other headphone to the balance monaural return,
or should we parallel the headphone transformer primary windings and
connect the incoming monaural hot side to one side of the paralleled
primary windings, and tie the incoming monaural return side to its
shield, and that then to the other side of the paralleled primaries
(converting the balanced feed to an unbalanced feed).  So far, we have
connected it unbalanced to the paralled primaries and found that it
works, but the audio level from the headphones is lower than we would
like.  It has been suggested that we insert a headphone amplifier in
the circuit, to bring up the level.  Our mixing board is an Allen &
Heath GL2200 and we're feeding the headset from the GL2200's Aux 2
output.