When a power cord could obviously affect SQ

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.

@ted_b 

Just recently while running 5Hz - 3500 MHz sweeps using a field strength instrument, I noted substantial differences in radiated EMI from various audio power cables. Some emitted surprisingly strong magnetic fields that radiated well beyond the cable jacket, whereas others had undetectable emissions. This implies some cables are clearly better suited for use in close proximity with sensitive digital components (e.g. DACs and streamers) and interconnects.

From an internal perspective, a power cable is not a one-way street, and the better way to view it is not as the last few feet of power feed, but rather as first few feet of what the component "sees" when propagating away problematic internal noise generated by the component itself. This noise exerts direct influence on audio performance, and cable characteristics determine how efficiently it is removed. When one views it from this perspective, it's not a stretch to understand why power cables can "sound" different.

I've quoted your entire post because it should be mandatory reading!  You have provided direct evidence for the theory that it is EMI coming from your component that could make a difference. 

@bruce19 many directly heated triode designs run AC to the cathode - straight from the transformer.