In Critical Listening, we did some intensive evaluation of what cable length does to sound quality -- both at line and speaker levels. The class found the damping factor demonstration to be particularly noticeable (how low DF drastically reduced bass punchiness). Everyone was able to quantify what they heard happening before we discussed the technical reasons (how it's not the cable that is coloring the sound, but rather the way the cable interacts with the electronics and devices at each end).
In this photo you can see the hundreds of feet of cable all strung together:
And the final test was AES digital audio at extreme cable lengths. Dropouts, jitter, pops and muting all came to the party!