"If you can determine that a tube was made in the same plant, has the same construction and date codes, how critical is it if the tube was made by Siemens and rebranded as an Amperex? "
only critical to appearance if appearance is critical
"I’ve heard people say that if a manufactures tubes are not up to their standards, THOSE are the tubes they send out to the other manufactures for their branding. Fact or fiction?"
nonsense AND fiction.
tubes are identified by and perform/sound similarly according to manufacture. rebrand is only as important as looks. even within one production run they vary in quality.
"matched," unless qualified by "gain matched" or mention of emmisions testing, refers to tubes of the same manufacture and contsruction testing within 5-7% plate current at spec voltage (power tubes) or 7-10% transconductance (preamp tubes) or both.
as far as branding/label, if you want them to look the same then hey it looks good. what someone writes on the outside doesnt alter origin, identity, sound or performance, nor does it opertain to matching.