A pre-amp should not add anything, warmth or "coldness". And the very good tube (and solid state) pre-amps don’t. If your system lacks warmth (whatever that means to you), better to address the source of that shortcoming. Complimentary colorations is a tricky business!
People choose tube pre-amps for what the good ones DON’T add to the sound of recordings ("etching", grain, brightness, etc.), and for what they reproduce that some solid state pre’s don’t (the organic timbre and texture of acoustic instruments and voices, depth-of-field, a "relaxed" presentation of music).
A very good pre-amp does not reveal it’s means of amplification (tubes, FET’s, transistors), only the sound of the recordings passing through it. That’s the basic and fundamental tenet of High Fidelity Music Reproduction.