Do good power cords have to be stiff?

Shunyata makes excellent power fiords and the have an EF (extra flex) version that works very well

Unless your equipment is very poorly designed, I don't think you should be able to tell any difference between any power cords, assuming a decent construction baseline.  I can't tell a difference in my system, and the only blind test that I know of confirmed my experience by a guy who has a million dollar system. . . But buy whatever you think will work for you. . .

@stereo5 The 7 gauge Pangea is ridiculous- had one years back. 

@tom6897 excellent point- many years ago I had a pricey balanced cable that was poorly terminated to the point that one leg broke off. I undid the Neutric connector, pulled back the tech flex, pulled back the company's own cable cover only to find that it was Belden cable. Nothing wrong with Belden, but it was a belden cable all dressed up.

Verastarr or other flat or ribbon cables for the win! Many round cables are so overly wrapped and shielded that they are practically unusable and surprisingly cover up generic readily available cable. Never buy without a right of return. 

Signal Cable.

most are but some aren’t

i use some excellent homebrew braided 3 conductor pc's that are uber flexible by a friend in the midwest - pm me if you are interested

The Shunyata power cords I use are very stiff, the Acrolink  power cord is very flexible.  I also have 3 of the 7 gauge Pangea power cords which are almost impossible to bend. It’s is weird that these Pangea cords work well with my speakers where their 14 gauge ones sucked the life out of the system. 

Transparent are relatively flexible (the box can prevent some degree, Cardas are very flexible. Both are also excellent. Cardas are warmer… too warm for most of my equipment. 

Puritan