Does a tube need the break-in hours?


Most probably not, but not sure why :) 
Does anybody know a good substitution for 12AT7WA tubes? I have bought 8 of them about 10
years ago for Manley Stingray. Now, when I need a replacement, none of them are good
(ones are labeled by Manley others are Philipps). 
(An almost 20 years old Stingrey remains in a good shape.) 
128x128niodari
Power tubes tend to sound better once played for a number of hours. In my experience that "number" varies greatly. I haven't found break in to be as true with smaller preamp tubes.  
in my experience short answer yes, but it varies wildly with diff tubes and diff brands