When I have one failed output transistor failed in a set I typically will replace the entire set. Not only that I will replace the drivers and pre-drivers that associate with those output transistors. You want to get it just working again but you want it to keep working for many years to come. There was one case where I had replace 3 output transistors on one side of a PA system and left the working originals in place. That amp still works to this day and that was over 5 years ago. In that case I was feeling mighty frugal and got really lucky. I still recommend against trying to get lucky.