I am not sure if HF cables behave similarly as some carbon nanotube Bybee filter cables I have, but have spoken to the designer about these.
I do know that for every current flow in a conductor there is an associated electric field and a magnetic field oriented at a right angle to the electric field. It is by manipulating either the electric field or magnetic field that current flow is altered. This phenomenon is well-known and it doesn't take much incredulity to believe that you can change current flow through the use of magnets. The trick that Rick has obviously found is through the orientation of the magnets to affect the current. Others have tried this effect with varying results, either by not affecting the current flow enough to produce a change in sound, or by magnetizing the cables over time to reduce the effect and the resultant sound improvement.
It is also well-known that electrons are normally found in pairs, each pair consisting of electrons having opposite spins, one of spin up and one of spin down. These packets of electrons are known as Cooper's Pairs of electrons. However, in current flow, some stray electrons are passed that are not in these packets. The designer of the Bybee Filter cables claims that the carbon nanotubes pass only Cooper's Pairs of electrons, thereby improving current flow. I suspect, but do not know of a surety, that manipulating the magnetic field would also reduce stray electrons and improve current flow. You can imagine that if you can manipulate and improve current flow that you could change the sound produced by a conductor.
Anyway, sorry for the technical lingo, but my attempt to explain what might be happening here on a quantum level. I wish I had stayed awake more in a quantum mechanics class I took in college. Whatever the explanation, I am quite taken by the sound of these cables once they have broken in.