I work in a pro audio shop as a service tech and i always touch up solder joints on things like transistors or multipin connectors whenever i am inside of an amp. it's just good practice, i also apply thremal compound where needed. that is something every tech should do just as good practice. the fact they look smaller is something i can't explain, just ask the repair shop and see what they say. i always include a bag full of parts i replace when working on a piece of gear just so the customer can see exactly what was changed. check the part numbers that is the best thing you can do, but i have never heard of anyone changing out parts only to take them. it's too much effort and time. there may have been an issue with one of the transistors and as you should do with any amp you change them all when one goes bad.