When a power cord could obviously affect SQ

Power cords or any upgrades never need to be a matter of belief. If the sound changes, it changes. If not, send the cables back. Morrow provides a 60 day return policy. The map4 power cord made an obvious improvement in my system. Will it in yours? It’s not guaranteed. Try it and find out. Return it if it doesn’t. Start with your power amp, then preamp. 

Going from stock cords on my power amps to heavier gauge/better insulated cords made an audible difference. Replacing the stock cords on my Streamer, DAC and Preamp also made a difference, just not as dramatic.

My experience.

Yep, experienced the same thing in 1998-1999. I bought an Arcam Alpha 9 CDP.

I had read reviews on the CDP at the time. I checked the demo unit out from the dealer for a few days to hear how it sounded in my system. Going from memory Stereophile gave the Alpha 9 a B rating. I can’t remember what the reviewer said at the time. Going from memory he said the bass was a little loose and highs slightly strained, for a better word.

The power cord that came with the Alpha 9 was a cheapo 16/3 power cord. Not sure why ArCam supplied a grounding type power cord. The unit has double insulated power power wiring. No IEC ground prong. 

I listened to the CDP for awhile using the power cord that came with the CDP. From listening to how the CDP sounded I was in agreement with the Stereophile Magazine review.

I kept thinking about that cheap power cord Arcam supplied with a $1500 CDP. (going from memory). I had the 14/3 OEM power cord that came my ARC tube amp I was no longer using. Boy did that power cord make a difference! The CDP has a max power usage rating of 20VA, 20 watts. 16/3 wire gauge was plenty big for 20 watts. 20W/120V = 0.166A. (Back panel lists 115V. 20W/115V = 0.17A) FWIW, ArCam used a massive size potted power transformer in the CDP... Why did Arcam do that? 20 watt load... 

Bass was notably tighter and highs were extended, not strained.

Why an OEM larger gauge wire made a difference? Beats me. There is not any cable build geometry used. Just three conductors twisted spirally together covered by a jacket. Nothing special about the end electrical connectors.  

Aftermarket audio grade power cords also made a difference in how the CDP sounded. I’ve never been a fan of shielded cables. Neither was the Arcam CDP. 

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Based on many of the ideas in this thread about the cable mitigating interference and such, it would stand to reason that you may not notice a difference if youre already using a real power conditioner.

Without getting into exotics, I do recommend the use of SHIELDED power cables after a power conditioner.  This will reduce the chances of picking up noise from nearby networking gear and cables as well as HDMI. 

Cheap one's are available from Amazon or build your own from DH Labs (affordable) or Furutech (not affordable):  https://partsconnexion.com/ac-power-bulk-wire-cable/

@retiredaudioguy 

He is supplying a pair of his preferred cables and he points out that the  2A3s’ cathodes are fed directly from the mains transformer.

It's a good sign he comped the cables to you for sure! I wonder if shielding is what he thinks of as important. Could well be. I'm a bit puzzled though about his concern about sensitivity to power quality coming from the mains transformer. That current should be passing though rectification and filtering capacitors to make nice clean, quiet DC for your cathod. If it is not it is a design problem, not something to be fixed with a power cable. Color me puzzled.