I have used those free 2m tiny toony wires, they worked for both for sub and digital coax connections. Yes they did work well until they broke. Only then did I replace them with heavier gauge shielded wires and better RCA plugs and found no noticeable sonic improvement.
Sub signals have a very limited bandwidth of about 15 hz to 120 hz, much smaller than the audio bandwidth of 20 hz to 20 khz.
Receivers typically output 2 volts "ordinary" AC audio signal at the sub out. Fed into a sub amp with a 47k input impedance, even at a maximum 4 volts, current is only 85 microamperes (4 volts / 47000 ohms). Also, with 2 volts pushing the sub audio signal, it is less prone to noise compared to the more common 200 millivolt line-out/pre-out signals. That's why even the "free" wires can easily carry the signals.
On the non-technical side, malaki talaga ang pogi points ng mahal na cable. But many don't care about this, tutal nakatago naman cables.