How tubes influence sound

From what I've read here it's clear that the general consensus is that tubes influence sound. For the purposes of this post I'm only asking about tubes in a preamp, mkay? And the tube pre only has 2 tubes.
Exactly how do tubes effect the sound? I'm not asking how each specific brand effects the sound, but in general terms. Do different tubes change the "color", "texture", or "feel" of the actual notes? Do different tubes go so far as to widen and/or deepen the soundstage? Are some tubes more "tubey" sounding than others?
Thanks.