Give the Coda Technologies FET Control Buffer O4r a try, although it probably doesn't fit the definition of a "pure" passive. c-j, Rowland, and I believe Levinson, to name a few are using similar technology and circuit design to the 04r's for the signal attenuation. In my opinion it's the best sounding way to do it. Regardless, I couldn't be more satisfied. Most whom have great passive preamps have found that the resolution, transparency & speed are unrivaled. You're on the right path, so long as your front end is up to the task.
Do you remember that "Aunt Corey's Passive..." article that Greenberg wrote for Stereophile? Well, this preamp has way more than a little in common with that circuit. Ask Eddie at Coda about that. Here's the link: http://www.coda-continuum.com/product/index.html .
