Hey, OOTD, looking good!
My thought is that the gnats and heat are causing the crispy leaves and not the light distance. I run my TSL 2000 QB about 18" - 24" from the tops of all my plants in veg and about 9" to 12" above in flower and have not had a problem with any of them getting heat/scald damage.
Also, I had some worms in a couple of mine that got pretty bad, (not sure if the were fungus gnat or not, no fliers), and I gave them a Hydrogen peroxide watering that knocked them all out in a couple days. The plants seemed to really like it too. I mixed about 1/3 H2O2 and 2/3 water and poured it in slowly until they were really wet with a very good run off and let it fizz for a bit.