ART Pro Audio CoolSwitchPro - Isolated A/B-Y Switch - Manual (Page 9)

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No smoke or sparks? Good. Now turn on the amps and turn up
the volume controls slightly. While strumming your guitar or
otherwise creating an input signal, verify that you get sound
from both amps by toggling the A/B footswitch. Toggle the Y
footswitch and verify that the sound comes from both amps or
just one. With sound from both amps, now's a good time to
listen for hum. If you hear any, try switching the GROUND LIFT
switch in the front to the LIFT position. If you still hear hum, try
switching the TRANSFORMER ISOLATE switch to the
ISOLATE position. For reducing hum, always try the ground lift
first, then use the transformer for isolation only if necessary, to
minimize the effect the COOLSwitchPro has on your signal.
Next, let's check that your amps are in phase, or, in other
words, that the speakers move in the same direction when
playing the same signal. As you might guess, the PHASE
switch is what we'll use to fix phase problems. However, note
that this switch only works when the transformer is switched in
(TRANSFORMER ISOLATE switch in the ISOLATE position).
Set up your amps side by side and set them for equal volume.
Make sure the Y footswitch is toggled so that sound comes
from both amps. While strumming your guitar (lower
frequencies or bass notes work best), try switching the PHASE
switch back and forth. One position should yield sound that is
fuller, more centered and with more defined bass, a sign that
the amps are in phase, The other position should sound
weaker and more hollow and unfocused, indicating the amps
are out of phase. Use the setting you prefer (you may like
hollow, weak sound).
That's all you really need to know to use your COOLSwitchPro.
However, here's a few more things to think about.