Both tubes or transistors can be Class A - it is a design choice. Tubes that have an output transformer are likely to sound the most different from s s amps (assuming a quality implementation of both and assuming no clipping)
Class A ensures no crossover distortion - it is inherently a safer design, however, a good Class AB power amplifier can be designed to run in pure Class A until around 50% power and then use Class AB in the final push pull output stages (this is both more efficient at high SPL'S where small cross over distortion is less important and good at low signal levels when crossover distortion is eliminated by Classs A operation)
Check out Chord amplifiers...
Class A ensures no crossover distortion - it is inherently a safer design, however, a good Class AB power amplifier can be designed to run in pure Class A until around 50% power and then use Class AB in the final push pull output stages (this is both more efficient at high SPL'S where small cross over distortion is less important and good at low signal levels when crossover distortion is eliminated by Classs A operation)
Check out Chord amplifiers...