Can a better power cable make a difference if NOT plugged directly into an outlet?

Yes

Thanks for some of the replies. 

I'm curious about trying the direct into the wall VS. via conditioner in addition to trying some power cords, in a careful, sequential fashion as described. I appreciate the factor Erik mentioned about interference as a key factor.

I have heard power cords make a difference -- in a store. I do not know what difference it would (or could) make in my system.

Remaining skeptical until proven otherwise is my M.O. for this hobby, because there are many other places to make improvements. So, if I don't hear something, I will send things back. 

@brunomarcs Regarding your comment: "You think this is a viable approach?"

Viable, sure. Optimal? Not exactly the order I would do it. I would want to go in the direction which is most likely to discover which components are most sensitive to certain types of cable upgrades (clarity in freq ranges of bass/mid-bass/mids/highs/ultra highs, extension in highs or lows, etc). So, I think it would bring you more understanding by starting the experiment with all freebie 16-18awg cables on all components and rotating all available upgrade options on each component, sequentially, component by component to find which component gains the greatest benefit from alternative cables you have available. 👍🏼 I think you’ll find this method efficient and effective.

In my digital system, I’ve consistently found that it’s best to optimize cables with the best high frequencies in the beginning of the chain, the best combo of high frequencies and mids on my DAC, something similar on my preamp, and make sure my power amp has a larger gauge to optimize lower frequency response and lowered resistance. For those with the budget, the Clarus Crimson High Current 8awg occ copper power cable is a fantastic power cable for amps, especially the more power hungry solid state amps.

@hilde45 When you go for it, a desktop computer with a decent power supply unit can be very useful as a cheap way to burn-in new cables before you try them out.

My computer has a 1000 watt PSU, so my m.o. now is to cook cables (usually in a daisy chain) alternating between 6 amps and 3.75 amps, continuously, for 2 or 3 days before I let them sit in my system with some ordinary usage for at least a day before starting to make decisions. That's a bare minimum.

Can a better power cable make a difference if NOT plugged directly into an outlet?

No.