Those bulbs give off very little heat, the actual surface of the bulb can get pretty hot, but the heat (and energy) are lost very fast.. therefore you really should have them as close as possible.

As for your watering.. it's kinda screwy. you say the soil goes dry enough to seperate from the pot?
That's to dry in my opinion, you maybe watering them until the water comes out of the bottom, but that's not to say that the soil is obsorbing it. i'd increase your watring frequency to every other day for a week, then try watering a little less..

with a fan in there, and i'm sure it's pretty warm.. the soil is gonna dry up pretty fast.

p.s what's the temperature at soil level? shouldn't be more than 85degrees.

The attached picture should give you an idea of how close you can get the bulbs, and the effectivness of them