Still there? I'm descended from the Kuhnau family -- this was the original spelling of the name when the first generation came over in 1848 but your branch seems to have changed it to Kiehnau. I also have the problem of multiple (mostly completely wrong) spellings of the name, but I have put together some things. Please get in touch and I'll share what I have, which takes your line back to Hesse-Darmstadt in Germany in the 1790s.
Email address is laukel AT waitrose.com.