I had 8 children, and know much less now than when i first started. But, i think if they are "generally lovely boys", you probably don't have that much to worry about. Over the years, i've gotten more and more laid back. I'm guessing that your intuition is correct, and they are just normal. I feel very uncomfortable around people who "super discipline/control" their children, and the kids act like perfect little zombies. THey're just human, they have moods, they get cranky, tired, sugared out, etc.
Sometimes if i see that pattern developing, of always fighting me about doing something, giving attitude, etc, i'll just calmly tell them that i'm adding on even more chores. That usually helps break the pattern, at least for a while. -- Often i'll say, you must do such and such, before you can do whatever (the more fun thing.) -- Sometimes i give them a bit of a break on being "mouthy", but other times i crack down, especially when i see a pattern, a habit of it.
Good luck, it's a challenge alright! but I'll bet you're doing great; you sound very conscientious and caring. :)
cheers, auntbee