Adam,
You are correct my friend and I was over simplifying the concept. I meant that they can act as tone controls in a very "broad" context.
As with most folks, I have my "boneyard" of cables that I've owned over the years. And I've listened to them enough, in various system iterations, to become familiar with their general sonic signature.
Example. If my system needs more top end extension with a more "analytical midrange" and less bass weight, I can substitute a Mapleshade IC, a DH Labs Silver based IC or a Kimber Cable PBJ IC and adjust the overall tonal quality in that desired direction. If I want a more toned down top with a fuller midrange and mid bass, I would install an older Tara Labs coax cable.
I obviously don't do that anymore but it is useful to help a friend set up a system and determine his or her biases regarding tonal "flavor". I've also used this to investigate / confirm differences I hear when I install a new component.
Hope that helps.
Best,
Gordon