Hi salty.
I have a theory. I probably cannot explain it well, but will try. You will simply put more dinos through the UV if going DT route. Typically, most of dinos in the system reside in the DT because that’s where the largest total photons of light are supplied and the most surface area. Assuming you have perfect circulation in your main tank and an infinite turnover rate to your sump, there should be the same concentration of dinos in your sump (neglecting the fact that many species only go into the water column at night and some not at all) . But we often don’t have that, hence the lower concentration in the sump. I have a low sump turnover of about 2 tank volumes per hour, so my sump trial was bad... There is a person recently that mentioned they had a 7 x turnover and he might have a shot. I need a good analogy for his, but haven’t come up with it yet
I know that some users of UV have been able to totally eliminate its use. I believe, and have read that you don’t need UV long term. Others, like myself, have figured that UV is somewhat an insurance policy. The strange thing is that my recent dinos certainly reared up when nutrients were low, by adding too much GFO combined with siporax and chaeto fuge. However, about 10 years ago, I had another bout of them that came about when my nutrients were high. I got rid of them back then by adding GFO!