"Have you got any ferrite rings? I am wondering what difference these might make placed around each end of a power cord."
It's already been done for you. Look inside your amp or preamp and at least one leg of the AC power cord has at least one turn through a ferrite ring or bead before the mains voltage enters the transformer. Feel along the cord that's powering your PC, laptop or whatever device you're typing on at the moment that isn't running on battery and you'll find a ferrite bead molded into the cord. Creates a bump in the impedance and resistance and filters out any RFI or EMI that the length of cord might have picked up. Any cord that is shielded be it twisted, braided, wrapped in foil or metal braid or is coaxial has higher capacitance, lower resistance and inductance. All power cords, ICs or Speaker cables incorporate these in one way or another. Litze, I believe is the most basic form and solid core is rarely used for countless other reasons.

