I have 6 foot ceilings in my basement as well, and run 30 1kw DEs down there now. The trick is short veg cycles, lots of small pots (we do 16 2 gallon plants per light) and prune, Phosphoload and bend to keep the canopy short. We get a little light bleaching here and there, but it's less than half an oz per 10 light crop. If I see no bleaching, our plants are too small or the bulbs need replacing, lol