The Stax SRM-400S is an electrostatic headphone amplifier designed to drive Stax electrostatic headphones. It offers a compact form factor and features a high-quality construction aimed at delivering accurate sound reproduction. The amplifier typically includes various input options, high voltage output for electrostatic headphones, and controls for gain adjustments and sound quality optimization. It's appreciated for its ability to deliver clear and detailed audio, making it suitable for audiophiles seeking an elevated listening experience.