I started with a Berning ZH-270 with no preamp. The Noble pot in the Berning was my passive. And then I started thinking about what a preamp would do to the sound. I then tried an Emotive Audio Poeta, and lived with that for about a year, maybe 2. I was happy, very happy. What that preamp did for my system was added richness, I thought that was it. It sounded 'glorious'...through the honeymoon period, which lasted a while. Then I became aware of the lump in the lower end, and how obtrusive it became. Then it sat and collected dust until someone bought it. Then I thought, perhaps I should look towards the more modern designs (the Emotive was definitely modern, it just looks vintage) in a more traditional sense - Hovland, VTL, etc etc. They weren't doing it for me out in the showrooms. It was only when I heard the Lamm system that I could not hear the gear anymore. The Lamm L1 was put in front of the Berning, and things started to trend upwards very quickly.
If you can find a Lamm preamp, it may be worth the effort. Any of them (used).