Uniti Atom Headphone Edition is a class-A amplifier that runs into class-AB when the going gets exciting. When it’s being used as a pre-amplifier it is pure class-A.
For example, when driving a 600Ω impedance headphone at full volume, the headphone amplifier will remain in class-A. At 300Ω at full volume the headphone amplifier will remain in class-A for all output except the last few As the impedance of the headphones drops then the point at which it slides out of class-A gets lower. Dropping to 32Ω it will be pure class-A until approximately 3.2mW and then seamlessly and transparently glide into class-AB for the remaining top 25dB.
Music is generally recorded around -20dB so for most usage Uniti Atom Headphone Edition can be considered class-A.