@Adam > the different outputs you are using have different levels:
Standard output level/Load impedance/Output impedance
MASTER 1 .............................................. +2 dBu/600 Ω/10 Ω or less
MASTER 2 .......................................................... +2 dBu/10 kΩ/1 kΩ
REC ..................................................................... –8 dBu/10 kΩ/1 kΩ
BOOTH ............................................................. +2 dBu/600 Ω/600 Ω
SEND ................................................................ –12 dBu/10 kΩ/1 kΩ
PHONES ................................................ +8.5 dBu/32 Ω/22 Ω or less