Where to begin? Let's talk qualifications. I have both a BS and MS degrees in electrical engineering from a major Big Ten University. I have over twenty years experience in the design, development, and analysis of exotic semiconductor devices. I am a coauthor on well over 100 papers published in refereed journals (most of them published by the IEEE).
You have made several statements that bear correcting, and to make things easy for you I will use layman's terms wherever possible.
First, electricity does not propagate at the speed of light. Only photons (little particles of light) can do this, as they have no mass. Like many of the math challenged here, you are confusing electricity with electromagnetic radiation. Secondly, the reason for only connecting the shield of STP --Manually Fix-- STEP STOP TSP SMTP ATP DTP ST SP SOP STA STE STU SAP SIP STY SUP FTP STD cable is to avoid forming ground loops. Ground loops are BAD THINGS that can cause systems to suffer from hum and buzz. Back EMF is a well-known phenomenon caused by any electrodynamic transducer returning to equilibrium from a physically displaced position (oops, big words, sorry). When a coil of wire moves in a magnetic field, a current is produced. This is known as a generator. So, when your woofer (the big speaker) is moving in response to a music signal, it is also generating a voltage similar to the input, yet distorted by magnetic nonlinearities. Distortion in an AC signal is commonly referred to as harmonics. I make no claims about the quality of the design of the amplifier in question. The troublesome cables were solid core type with separate feeds for positive and negative, and thus more inductive, while the ones that worked were more capacitive (Joe, I know you like resistance, but there are other passive forms of impedance, it's just that they are associated with scary imaginary numbers). I haven't performed a thorough analysis of the system. I don't own a personal copy of PSPICE, and can't in good conscience use work resources for such a personal activity. I do, however, have a number of engineering textbooks in my desk, but can't mention the titles due to the large number of big scary words.
A bit of advice. Most good scientists are well aware of the fact that our current stage of knowledge is base on theories, which are simple mathematical models of reality. Those of us working in the field are excited to discover cases that don't seem to make sense. It allows us the chance to possible get a little closer to understanding the way things really are. It's people like you who know that heavier things fall faster than lighter things that contribute to the slow growth of wisdom.