Vacuum tube preamp?

I have read that tube preamps can add some "warmth" and perhaps "soften" the audio signal input to the power amp. This has been recommended for systems which seem a bit "bright" in the reproduction of the higher range. I have Thiel CS 2's driven by a Hafler 9180 power amp and a Rotel 971 preamp. I haven't noticed feeling fatigued listening over long periods nor do I hear grain in the highs. I also wonder about characteristics of a given recording as the culprit more than my equipment. Any thoughts? If I did replace my preamp with a tube model I would want one at $500 or less? Do they exist at this low price?