I did the same thing recently after being a SS guy since the purchase of my first system. I was curious about all the fuss around tubes but at the same time didn't know what to expect or what to look for. After careful thought and many reviews, I've decided to go for a Conrad Johnson. One of the best decisions I've made with audio. Mainly because of the value, reputation and general availability of their gear.
CJ offer preamps in different price levels. A lot are available on the used market. I think it is mainly because you can upgrade your CJ for a better CJ without taking a new mortgage or divorcing your wife eveytime. A lot of people do this without changing for another brand.
Whatever the model, there is always somebody, somewhere who is going to come out with a better preamp for the same amount of bucks. This is called competition. But as a first experiment with tubes, a CJ will not let you down being simply for your listening pleasure and/or the possibility of trading it quickly for an upgrade.
Good luck !