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 
12 series tubes should be plug and play.
12ax7s should last 2000-5000 hours.