No, it is all about the iron used in those old transformers.
I read that theory once. New iron is bad sh... stuff. Then one of the old Neve/St. Ives engineers joined in said that they bought scrap iron as cheap as possible. That new iorn is actually a lot purer.
Maybe all the impurities gave the old transformers that vintage grunge factor? Fun stuff. Now, how many angels can dance on the head of a pin?