Why it is unlikely the AUX and MAIN have the same phase?
Signal paths are likely to be different; any filter modifying the bandwidth and EQ of the signal is
also affecting the phase.
Example: a 24dB/oct high pass filter set at 15Hz is only affecting amplitude of the signal by
0.6dB at 30Hz but the phase shift is 90°!! At 100Hz we can still measure 25° of phase shift
Should you want to restrict the bandwidth with a low pass filter, you can introduce a phase
difference of up to 180° (completely out of phase) at the cross over point
If the signal is passing though any digital equipment you are adding between 1.4ms and 2.2ms
(around 70° phase shift at 100Hz) due to the converter delay only! The additional delay due to
the processing itself (look ahead compressor, delay ) can be quite important
(Kaynak: http://www.nexo-sa.com/asp/catalogue/do ... ish_French)