Tubes are cool but are they necessary?

"Rich, warm tone" is a consequence of elevated mid-bass response, attenuated high-frequency response, or both. Tube amplifiers have a reputation for warm sound because they tend to have high output impedances (low damping factors), which can cause their frequency responses to bump up at the connected speaker's bass resonance frequency. (I think the fact that they glow in the dark also has an influence, but ...) You can get the same result with a tone control or equalizer, however.