Dear Yogi,
You've never heard of UV meters? You have NO idea how much UV depreciation your "bulbs" have at 600-700 hrs let alone 1100 hrs. They might already be down 25 or 30% in output.
When you finally decide to re-lamp... get a 5.0 meter and start tracking aging from new value. Or put two new ones of the same kind in NOW and check them vs the old ones to see difference.
Welcome to Solartech