So you can tell a difference between two pairs of the exact headphones but not tell which is the new one?

Yes, that’s how it works. If you had 2 pairs of the same headphones or speakers you would know.

Just recently got them at a discounted price. I also have the open Sundara and the LCD-X.

I took the closed ones over to my girlfriend’s place so I don’t bother her with open cans while she works and the closed ones are about the same as you would imagine CLOSED Sundara to sound. I haven’t heard anything better at this price. After a few hours of burn-in they don’t bother me at all.
I run them with either Fiio K7 or the Zen stack.