My experience with the big UK chains is that in the past [if not still now],
they insisted that if a product is defective within a stated short period after purchase,
they will exchange for another brand new factory boxed replacement.

Beyond that period of a few weeks they will only offer repair under the one year warranty..

I'm not sure if this is legally correct, or if they are just trying it on to limit their responsibility and losses.

In the past 10 years I have experienced 3 or 4 brand new valve amps fail shortly after purchase.

Amazon however, have been really good & positive, and refunded on return of faulty product
right up until the 12th month of one year guarantee.