I don't like the term "warm". I think it is a euphemism for bland and muddy. It is generally an attempt to fix a too bright system. and often the too brightness is really a lack of bass. People don't understand this but it makes sense if you think about it. For example, you have a crappy power conditioner that limits current to your amp that mostly affect bass. So to get the normal bass and volume, you have to turn it up. now the highs are screaming at you.
So instead of buying a "warm" preap, I'd try to find out what is causing problems in my system.
Jerry