I wouldn't try to tube the Holston. There are multiple ledges, drop offs and just plan dangerous spots, not to mention the long and I mean long flat water. A person could be down right miserable before lunch.....if they lasted that long.
Take a boat or hire one of the local guides to take you, but I would be afraid to do any of the access points to another access point in a tube.
It is a fun river in a boat though, that's for sure!