A/C power line polarity on new phono preamp

Getting ready to help a buddy finish setting up a new phono preamp over the weekend…he’s not sure about the A/C power line polarity (rear panel switch allows “A” to “B” switching for best performance).   Any suggestions on what’s best to listen to as you switch from A to B ?   Certain kinds of music and/or test tones best for this?   Any specific suggestions welcome.   
Get a volt meter, move the phono stage over near to the wall socket, plug it in, measure from the chassis to the center screw of the AC cover, note the reading. Reverse the plug and measure again. Compare the two readings on the volt meter. The lower reading is the correct orientation of the AC plug. If the phono stage plug is polarized (one blade wider than the other) you will need to use a two-prong cheater plug to make the voltage readings. This is the proper way to determine the plug orientation!
You are measuring eddy currents on the chassis. The center screw on the AC outlet cover is ground. Less measured voltage means less eddy currents and a lower noise floor.
Since AC plugs are almost always polarized you will have to leave the cheater plug in place for the proper orientation.
You can do this for all electronic components. This is the best way to achieve the lowest noise floor for the whole system. Listening tests are not accurate, so get out that volt meter!
Blah blah. I see no mention of plug orientation in the OP.
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@jasonbourne52 - thanks for the recommendation, I have a voltmeter (somewhere) in the garage (and a cheater) I’ll dig them out.   

@tvad — the html link didn’t work for me, can you re-post what’s on the other end of that link

@fuzztone — I haven’t scoped out his A/C outlet orientation/situation (yet), I’ll know more once I get over there this weekend 

Thanks guys (and gals??)…appreciate any & all input.  
Best regards,
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he’s not sure about the A/C power line polarity (rear panel switch allows “A” to “B” switching for best performance).

That’s odd the manufacturer would provide a switch for switching the AC polarity of the primary winding leads of the power transformer. If the manufacturer first checked the primary winding of the transformer for the proper polarity orientation before wiring the primary of the transformer into the circuitry of the preamp why would he add a switch?

What’s really going on?
A quote from the late Charles Hansen (M)

The other side of the coin (long) 1) Reversed AC polarity -- All power transformers have an inherent asymmetry to their construction. The primary winding comprises multiple layers, so that one lead is connected to the innermost windings and the other lead is connected to the outermost windings. This means that one lead has a higher coupling capacitance to the core of the transformer. Please remember that the AC supply is also asymmetrical, with the neutral lead essentially being at ground potential (assuming there is not a fault in the house wiring). The result is that one orientation will give a higher AC leakage current to the chassis of the amp (and worse sound) than the other orientation.

Not all transformer manufacturers use consistent markings on their transformers so that the correct orientation can be identified, and not all amp manufacturers pay attention to this even if the transformer is correctly marked. The result is that many audio products have a random chance of being correctly oriented. I would have to assume that the amp was modded to achieve the correct orientation, thereby achieving improved sound quality.

The correct way to do this is to swap the transformer leads, and not the connections at the IEC connector. The difference is that changing the leads at the IEC connector will move the power switch and fuse from the hot side of the line to the neutral side. While this *could* increase the liklihood of shock under a very unlikely set of circumstances, in my opinion the service technician *vastly* overstated the hazard that was introduced by this mod.

Read what John Curl has to say on the subject...

If you perform the AC polarity orientation test the preamp must be isolated from all other audio equipment. Therein nothing connected to any inputs or outputs on the preamp.

Chances are the cheaper plug you use will be polarized. Therein the neutral blade on the plug is wider than the Hot blade. You will need to trim down the wider neutral blade with a pair of tin snips so you will be able to reverse the plug in the wall outlet.

You will to need to insulate the bare ground tab on the cheater plug so it does not come into contact with the grounded trim screw that holds on the wall outlet cover plate. Any type of non conductive tape will work.

FWIW... Not only can the wall outlet be wired wrong with the hot and neutral conductors reversed, DIY power cords can have the plug to IEC female connector reversed. Even power cords sold on the retail market can be/are wired wrong. Not the big name brands but those made by some guy making then in their basement or garage where the final assembled power cord is not checked with a meter or a built testing device. Several years ago there was a thread running on Audio Asylum where some of a guy’s power cords had the hot and neutral reversed.

Usually the miswired mistake is found at the IEC connector.
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I'm pretty sure that switch is for signal polarity, not AC power.
A simple 3 prong plug in device with 3 leds.  The leds indicate the polarity status.  A fast method of determining the polarity of a wall plug.  Had a Sears, now use a Harbor Freight brand.  It is called a outlet circuit tester model GFCI.
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I have never bothered with checking the AC plug orientation. The difference is so small that it is inaudible to me.
@gs5556 : I think you could be right! That switch could be for signal polarity!
The difference in eddy currents from AC plug orientation is usually too small to worry about. 
Try both switch positions and choose for yourself. If it’s switching AC polarity, you don’t need no stinkin cheater plug. Phase switch for audio signal is usually on the front panel not out back near IEC. Anyway, is there no owners manual?
Chaps, equipment power polarity matters. I've been checking and altering it since the 1970s, particularly with monoblocks. Manufacturers don't always measure their transformer polarity and there can be discrepancies. I once got a 250V CT transformer that was symmetrical about the CT from a manufacturer that claimed 100% goods outwards test.

This image shows what happens when polarities are inverted with monoblocks.

This link gives an example of miswired power transformers: ieLogical VTA M-125 HowBad - note the difference in the noise with the polarity reversed.

Articles on AC Polarity:
Transformer Polarity | Energy Central

When AC polarity is correct, another of the proverbial veils is removed.

To make a polarity check adapter, get a cheater plug with a ring Earth Safety terminal. File down the neutral so it fits the line. Solder a piece of 12ga stranded to the Earth Safety tab and add a large insulated banana plug to the other end.  4897-0 Pomona Electronics | Connectors, Interconnects | DigiKey Expand the leaves slightly to ensure a good connection. Plug the banana into the neutral on the other socket. If it's used, temporarily use a cube tap to increase the available outlets.

NEVER EVER lift the Earth Safety. FIX the BLEEDING PROBLEM!!!
The life you save may be your own.