Do good power cords have to be stiff?

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. 

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. 

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

Signal Cable.

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.