Probably no chance of bringing this conversation back-on-topic at this stage, but this idea that solid wire is "the worst" needs to be taken with a grain of salt. Like, the fact that they use tubes of copper instead of solid bars is *not* because the tubes are better than solid bars in any absolute sense; they're just better per kilogram of copper. Carving the copper out of the middle of the rod to make the tube (yes I know that's not how tubes are made) increases the AC resistance a bit, they only reason they do so is because it's worth it for the copper your reclaim.
Consider comparing 1.5mm solid wire with 2.5mm solid wire. You would think that because the 2.5mm solid wire is bigger, the proportion of metal that is at the surface (by whatever fixed definition of depth you choose) will be lower, and that the "skin effect" would be more severe. Indeed, the ratiometric difference between DC resistance of 2.5mm solid wire and 20 kHz AC resistance of 2.5mm solid wire is greater than the ratiometric difference between DC 1.5mm and 20kHz 1.5mm.
HOWEVER, the 2.5mm solid wire has considerably more surface area than the 1.5mm solid wire, so its AC resistance at any given frequency is lower. It's just that at the limiting case, it decreases in proportion to the radius, not the radius squared as you would normally assume. The obvious and intuitive fact that 2.5mm solid wire carries electricity much better than a 1.5mm conductor holds perfectly true at both DC and 20kHz.
Take some speakers 10m (!) away from the amplifier, so a loop length of 20m. Here are the impedances at DC and 20 kHz:
DC 20kHz
1.5mm 0.190 0.215 ohms
2.5mm 0.068 0.111 ohms
Now assuming the speakers have a flat 8 Ohm resistance (actually have no idea how valid this is but let's run with it), that works out to attenuations (in dB) of:
DC 20kHz
1.5mm 0.204 0.230 dB
2.5mm 0.074 0.120 dB
To my surprise, it turns out that the flatness of the 2.5mm wire (0.046 dB) is inferior to the 1.5mm wire (0.026 dB). However, let's just keep things in context: 0.046 dB of flatness is absolutely inaudibly insignificant. Remember we're talking about a 10m long speaker wire here.
Reflecting upon this further, it turns out that both extremes (a tiny hair of wire that is immune to skin effect and a solid infinite diameter ingot of copper) have perfect flatness. There exists a worst-case diameter for flatness from DC to 20kHz, somewhere above 2.5mm. I wonder what that is?
Either way, avoiding solid wire on the basis of "skin effect" in audio applications is just silly.