The Alto Professional HPA4 is a robust, 4-channel, 1U rack-mount stereo headphone amplifier designed for studio and live use, offering independent mixes, level/tone controls (bass/treble), AUX inputs, and multiple headphone jacks per channel for up to 12 listeners, providing versatile personal monitoring for musicians and engineers. 

The Alto Professional HPA6 is a compact, 1U rack-mount 6-channel stereo headphone amplifier and distribution system for studios, stages, and live events, providing six independent, high-power headphone mixes with individual level controls, aux inputs for personal sources (like click tracks), and multiple headphone outputs per channel for up to 18 headphones, ensuring clear, crisp sound for musicians and engineers.