Is it worth it?

I just bought a Bat VK75 and the gentleman
offered an additional set of tubes for 400
dollars.He feels they make a big improvement.
These tubes have low hours and about 1/2 price.
[2 mullards and 6 RCA's] Should I buy them?
I just spent a bundle to retube my Kora Pre and monoblock triodes with cca 6922's. I am loving it!
BE CAREFUL! If you don't know their authentity than don't buy and spend more to buy real.