I would add to audiotroy's list:
1. Dynamics--tube gear sounds more alive and dynamic, particular at low volume. Solid state has to be cranked up much louder to come alive.
2. Natural/realistic attack/transients--the initial attack of a note sounds more natural and not as artificially hard or brittle.
Many of the tube amps that have XLR input jacks have them for a convenience for people with balanced cables, but the circuitry is not fully balanced. If you have balanced cables and a piece of gear does not have XLR inputs, you can buy adaptors so you can keep the balanced cables. This really should not be a limiting factor because the vast majority of tube amplifiers are not fully balanced.