Ideally, one would install impedance matched transformers at either end of the cable to boost the peak voltage along the transmission line to something near the 200kV range and take full advantage of the skinning effect, thereby taking the composition of the wire strands, flats, twists, and even the metallurgy completely out of the picture. There can be no doubt if you have a few hundred kV running your speakers that you are the BEST!
Short of that, 49 strand nickel plated is far superior to 7 strand bare copper, but make sure you use phosphor-bronze brazing for the connections instead of tin/lead/antimony solder.