Grounding Speakers

I have the tannot legacy eatons and I use 12 gauge stranded green wire (could use solid copper, but a bit less flexibility), terminated to one gold plated 8 gauge #6 spade to the amp. On speaker end, stripped and silver solder tinned wire ends to the 5th ground speaker terminal. The ground wires from both left and right channels are terminated together into the one aforementioned spade connector (two 12 gauge wires equates to 9 gauge) and attach to one of the rear chassis screws of my Sugden A21se. Not certain if I can detect a difference....anyhow, I like the fact that the 5th terminal is provided and it probably can’t hurt...

Yes supposedly over time, stranded can develop the so called skin effect distortion etc, at least that is what I've read. Stranded major advantage is flexibility. Solid is a bit more difficult to work with, but is better for a more wide open space, as stranded is better for tighter quarters due to its flexibility. However, it being only the ground wire and not a signal carrying wire, it probably will not have any audible influence whether it be solid or stranded wire.