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?
The best thing would be to find someone local that will let you audition one in your system.
I agree with Jdombrow, you should try borrowing one and see if the sound works for you. The "warm tube sound" is a generalization. There are incredibly transparent tube preamps, and very warm,rich, syrupy ones. And everything in between.

If you don't have any complaints about the sound now, what is it that you're trying to improve on?

Some options in about your price range that I think are pretty good for the price include the Marsh P2000t and Audio Mirror T-61.

Agree with the above. Tube preamps (and amps) cover the gamut from transparent to extremely colored with all shades in between. Better to try out a few and figure out what sound you prefer. Otherwise you will just get a bunch of recommendations that might not fit what you are going for.
A cheap tube preamp is going to lose resolution to gain some coloration which is generally liked, and called warmth.
One additional help is it cuts the high frequency digital grunge.. smoothing it into something reasonable.
IMHO, the Thiel CS 2's are best driven with a warm ss amp. With that said, as suitably compatible tube pre is a very good idea. With your budget I would consider a used conrad-johnson PV 10 BL. Which BTW would work very nicely with a used conrad-johnson MF series solid state amp and your Thiel CS 2's.
A correctly modded PAS 3 sounds about as good as anything.
dared sl-2000a can't be beat in that price range

I'm not familiar with your Rotel pre, but I had a Rotel RC1070 pre and while cheap, I wasn't hearing enough differences with other pres that would make me want to change, until I got a Lamm LL2. Certainly I can hear that difference, but the Lamm is like 10x the price of the Rotel. Point is don't assume more expensive will always sound better.
Hi all ! I have had 2 pairs of the CS2's . They are the easiest of all Thiel speakers to drive , so dont limit your choices on the preamp side . A Belles a150 will drive the crap out of those and can be had for $400 - $500...this amp is nice and warm sounding . Dont know much about preamps ,I have used the placette passive and a cary slp-03 for years and both were are excellent. PS... also a nice 40 - 60 watt tube amp would be a good match too.