As usual I am going to be very blunt in my answer. Keep in mind this comes from 10 years of guiding on the Holston and the only person who has probably done more trips on the river then me is Mike Bone.
First, take every caddis pupae, emerger, larva, etc etc that you have in your box and throw them away. They will not consistently work on the Holston for some unknown reason. I cannot count the number of days I have had clients catching fish which were puking up caddis emergers/pupae and decided to try one of 1000 different patters to no avail. Switch back to a PT and destroy fish on almost every cast. As I told them each time, we don't make the rules, and the fish certainly don't play by them either.
Yes the Holston fish eat tons of caddis, and eat plenty of dry caddis as well. The issue becomes presenting caddis dries properly. If you watch the fish on the Holston feeding on caddis, they primarily key on the bugs which are moving. Which is common for caddis feeders. Now try to imitate that, and good luck.
If you want to be successful on the Holston, throw out the stuff you think should work because it is what the fish are feeding on, and drop back to simple stuff. Dark simple bodied nymphs (PT's) in 16 & 18. Leave the rest of it alone. Anyone and I mean anyone who tries to convince you they are having success on one of the 8 million caddis larvae is either trying to make money selling you flies, or lying. Period.
If you really really want to fish dries, tie or buy some 16 & 18 Griffiths Gnats, and nothing more. They will work, trust me **** near every fish will eat it if presented properly.
The Holston is by far the easiest tailwater to be successful on day in and day out. Just keep it simple and don't get duped into over complicating it.