He just switched the power phase and amp changed sound drastically.
The AC feed to your house (in the US) has three wires.
120vAC referenced to ground
120VAC referenced to ground with polarity opposite to the other feed - 180 degree phase difference.
0V Ground Return
The out of phase 120VAC provides 240VAC when the load is across the two phases, for kitchen range/oven/clthes dryer, Central Air conditioner, and any other high power applications.
At the breaker box, half the breakers will wire to one 120VAC line, and half to the other.
Your gear shouldn't care which "phase" it is connected to. They are the same, unless compared to each other, like a mirror image.
Why it "sounded different" would be a mystery, shouldn't be related to which 120VAC wire from the transformer outside feeds the gear.
Shouldn't transformer cancel any phase issue in the power?
The gear transformer only sees 120VAC 60Hz on either feed.
I would go "Uh huh" and move on.
Salesman tried selling me expensive cables.
Uh huh.
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There
might be an issue if some of the gear is on one breaker (and phase) and some on the another. Maybe . Ground noise? Maybe. Probably not.
Or he has something interesting about his power at the store.
He then rotated cable and sound was way different.
Different how?