It will always be a combination of factors that show if your boys are happy/content. If you think about humans, some humans cry when they are sad, some don't, some cry when they are happy so crying in humans isn't necessarily a sign of unhappiness and not crying therefore isn't a sign of happiness, do you see. Same with cats, some cats who are in pain or who are really ill especially chronic illness can and do purr on occasion, this is usually something that people think of when they are trying to decide when is the right time to PTS, it can truly confuse and break your heart if your cat who you have realised has very little quallity of life and is plainly uncomfortable, purrs when you give them attention, that's when you need to be aware that purring does not necessarily mean that the cat is well and happy.
If your boys eat and sleep and play and have bright eyes and a good coat and spend time with you then it sounds like everyone is doing just great!
If they shy away or go off their food or show other signs or symptoms of ill health thats when you need to worry.