OTOH - there may be something into this tonal thing, as I can see that solid core wire may lend itself to other wire routing schemes, that just may result in a lower signal-ground and/or interstage capacitance than a loom of stranded wire. Given the higher impedance nature of tube amps, this may be audible - I don't know....I haven't built a guitar amp yet, so you tell me.....
As it happens, I actually work in the rocket bussiness (!) myself ( now understand my nick ? )and have made a few harnesses, too - I've never seen a solid core wire in that context, and I'd never use solid in my own builds, except maybe for the odd ground bus or so ( or maybe for a few other tasks I cannot think of at the moment) .
EDIT: and yes - the standard wire is mostly stranded teflon, for reasons of temperature, voltage rating and vibration. Sometimes there are other variations for specific reasons, mostly being screened wires or coax....