There's lots of theories, of course.
(1) on the source, since garbage in = garbage out. Or in other words you can't change the signal for the better later on in the chain if it's not as good as it gets right from the start.
(2) on the preamp, since that is the hub of any system and usually has the greatest effect on the tone and feeling in your system.
(3) on the amp, since that's what needs the most and the cleanest power
(4) Bob Crump used to say on your turntable motor, since that critical and sensitive component has the most impact on your vinyl pleasure
I would actually say, whever that particular cord sounds best, because the same cord can definitely sound different on an amp or a source component. Sometimes making things better in one place but making them worse in another (to your ears, of course).
Enjoy,
Bob