OP,
Audio Research has chosen relatively new, made specifically for high end audio tubes. So these are not tubes made decades ago and have all sorts of variations. Personally, I buy Audio Research to sound like they intended it to sound like, not all sorts of variations. But their are lots of folks that love playing around with the sound by changing the brand of tube… to something made in the 1940’s or some new gold plated thing out of China.
Personally I am not looking for greater variability, I am looking for the right sound for me and Audio Research with the tubes that come with it are what I like.