Soundstage, layering and tube differences

you call burn in whatever you want

any electrical device, connection, circuit, will ’settle in’ over some time...whether it is seconds, minutes, hours, days... it has to do with physics of matter, electrical flow, heat/heat dissipation

w.r.t. tubes, they definitely have different sonic characteristics (frequency, amplitude, and phase differences)... not to mention your equipment interacting with the tube (driver and buffer circuits, components within those circuits), transformers, power supplies... each design/component operates as a system... nothing works in isolation