Rob - Thiel's wire was developed in the late 70s through our contact with an aerospace avionics engineer (cousin Ted). For many reasons we settled on OFHC polished solid copper in teflon jackets with a tight 3/inch) twist. Wire carries competing parameters, but that configuration does more right and less wrong than most other options - and we landed there.
In developing the CS7 / 6 (after my time) Jim reportedly revisited wire and kept the original configuration, although the 5-9s aerospace ware was no longer available. Best of form is CDA101 @ There are differing opinions, and I am looking closely at options. There is justification for not going to thicker gauges in the lower frequencies due to varying skin effects. Also, many of his networks see a series coil as the first element and it might present hundreds of feet of wire and were rarely larger than 18 gauge, sometimes smaller, which would make larger wire meaningless. My present study suggests that larger gauge coils in some of those elements would be desirable - because there is more going on in wire than plain resistance. 99.99%, etc. Ours is slow extraction, high polish, etc. Straightwire supplied that from the mid 80s to the end.
I am moving toward star quad for noise suppression. Tweeter is double twisted (star quad) 18 gauge as described here before. Midrange might be the same unless I insert a 16 gauge foil coil in the first midrange element, then it will be star quad 16s. Woofer might end up being star quad 14. But that's unsettled and dependent of crossover variables. But all remain solid, not stranded, and that opinion is well corroborated in the technical field.
Anyone who says that only the big three: resistance, capacitance and inductance matter just hasn't looked deeply enough.
As a historical aside, I've mentioned here before that I did some work with John Dunlavy when he was moving to Colorado from Down Under. John's specialty and multiple patents were in the field of antennae, especially deep space low signal communications antennae. He played his cards pretty close to the vest, but he paid extensive attention to wire and wire routing within his speakers with knowledge of propagation interference and conjugation. I wish I knew a fraction of what that guy knew; but I am asking more questions in that realm than Jim did. And I hope to (dare I say expect to) reap some benefits.
