The tops were not being burnt because of too much light; they were being burnt by the heat generated by the lights. This stresses the marijuana plant and causes the tissue to burn. Your lighting input was around 67 watts per square foot (psf), which is not an excessive amount of light, particularly if the air was enriched with CO2. Taking the wattage down to 33 watts will adversely affect the growth of the buds. The amount of tissue growth and development is directly proportional to the amount of light input.
One easy solution is to raise the lamp so that there can be more ventilation underneath the lights. Ample ventilation is good, but it won’t completely cure your ills because a large proportion of the heat is emitted as infrared radiation. When the infrared light hits the leaf, it turns into heat.
You could also use air- or water-cooled lights. Air-cooled lamps ventilate the lamp with a steady flow of unheated air supplied with an intake tube and then expelled from the growing space with an outtake tube. The reflector holds a sheet of glass that keeps the air enclosed in the system. Water-cooled lights basically do the same thing, but are more efficient because of water’s higher mass. With either one of these cooling methods, you can place the lights closer to the plants to maintain intensity.