Think about it. You use a 40 watt amp on volume setting 2 and it's loud, so if you change to a 30 watter, you can crank it more, get the tubes hotter and it will sound better.
Saying that, i don't think there's a huge difference, you won't be able to like, turn the 30 watter to 11 or anything, but you can crank it more.