This bundle includes the HP4 4-channel headphone amplifier and the HD9 professional monitoring headphones from PreSonus. HP4 - Tired of straining to hear the headphones? The HP4's four 130 mW headphone outputs are clear, clean, and screaming loud, all packed in a compact, 1/3U chassis. Features include balanced L/R line inputs and outputs, global mute, overall level control, and sum to mono. The line outputs can feed another HP4 or an HP60. HD9 - When you need to do serious work without bothering anyone in your living space and without being distracted by outside sound, choose PreSonus HD9 headphones. They're clean-sounding, extra-comfortable, and offer a broad, transparent frequency response that ensures accurate mixing. The HD9's closed-back acoustic design provides superior listening isolation and reduces headphone bleed, so you hear nothing but music - and your housemates hear nothing at all.