Phase shift due to different crossover networks in speakers ? Some speakers in
back have switch adjustments for cutoff which affect phase at speaker. They also
would affect amplifier loading and possible phase shift.
If you need to build a phase shifter network allpass would be the ticket -
Two wideband transconductance amplifiers produce an allpass network. Combining two such networks as shown in the illustration produces two outputs with a constant 90 degree phase shift (vs. frequency) between them.
In your scope, "Acquire" button, "Mode" then set it to "Average", adjust "Averages" setting to 64 for starters.
That will get rid of noise.
Regards, Dana.