I am coming into this discussion at a late date,and I am glad I didn't see it years ago and get tempted to build a new UV box with LED's..
About 1990 I built a box with 10 - BL bulbs. It has functioned well ever since and I have never replaced a bulb. There are 10 bulbs as close together as I could get them. A muffin fan cools the unit. My exposure times are about 2-3 times longer than the same print in the bright Southern Cal sun. I also have a NUArc, but i prefer the BL tubes. They are economical and do not heat up my darkroom as the NuArc would if I had it in the same 100sg.ft space.
My tubes are approximately 1.75" above my negatives.
Several years ago an experiment was performed by Judy Siegel publisher ofthe "The World Journal of Post Factory Photography". She was attempting to discover if the output of fluorescent tubes was evenly distributed. She blacked out every other tube in her UV box. Of course the exposure times were longer, but not doubled. There was no banding. Then she laid the printing package directly on the tubes. Still no banding. Why is this important?
Because it shows that fluorescent tubes are so diffuse they will expose our prints evenly with no banding even under unusual circumstances.
Until LED's can match the diffusion, UV output, long life-span and other qualities of BL tubes, I'll stay with what I have. AS I am now nearing 91, I don't expect to see it.