Brooder setup. Just gave it some thoughts, because you've said that your temp doesn't get that high up. I would go more by the ducklings behavior. If they huddle tightly together then it is to cold. It they move around and just lay close to each other then you are fine and I would not even try to get it hotter. You have the setup in your house, no big temperature variation between day and night, so they should be fine. Plus the one duck did fine the first night. Larger breeds can usually live with less heat then small ones. Keep on doing what you are doing. I currently have 3 week old ducks that sleep in the duck house at 65 degrees at night. Technically they should be at 75, but they are doing fine at the lower temp. I simply keep an eye on their behavior, and they show no sign of being cold. I went down quickly with less strong light bulbs. Didn't even have to switch weekly, it was more like every couple of days. You will notice that they will move away from the lamp as they need less of it.
Katharina