Aloha e @BethKing-M, I would think that kī is the state of being in tune, or having been tuned up. Hoʻokī is the causative form, and would imply "to cause to be tuned" or in English the verb form "to tune something." This is my guess based on knowledge of other Hawaiian verbs/statives:

In past tense, use kī. In other cases, use hoʻokī. Ex: Ua kī ʻo ia i ka ʻenekini o kona kaʻa. He tuned (up) the engine of his car.

E hoʻokī ʻoe i kāu kīkā. You should tune your guitar.

Hope this helps! If anyone can comment further on this, please do so. I would like to know as well, but this is how my brain feels it is lol.