It's not just the electric guitar and it's associated loudspeaker, but the impedance loading of the input stage, the gain stage and output stage. Each amplifier has it's own coloration, especially tube amps.
Guys who play with solid state amps can easily adapt to using a Pod product, for instance, because they are familiar with the solid state sound.
Guitar players who have grown attached to the tube sound, well, need a tube amp with a tube input stage and a tube output stage.
Trouble with the tube is, it's difficult to find a tube amp that doesn't require high gain to get the sound the guitar player is familiar with.
I'm searching for a thread from another site regarding low wattage all tube practice/stage amps.