The Grace m903 is a fantastic combination of headphone amp and monitor controller DAC, featuring just about every analog and digital connection type possible: AES, S/PDIF, TOSlink, USB, RCA unbalanced in, XLR balanced in, and two stereo outputs. The fidelity is excellent, and rivals more recently released top-end gear. Please see the text towards the bottom for even more specifics.

This particular model is in excellent condition, and used gently in my smoke-free mastering studio. It was sent back to Grace Design late November 2024 for a full check up, including a power switch replacement, so the unit is essentially brand new. Please see photos for some very minor cosmetic issues. The m903 will ship with a power cable, and also comes with its own infrared remote. The remote is very hard to come by on the used marketplace, as Grace has since discontinued it and have not made other compatible remotes. It breaks my heart to part with this; it is one of the most satisfying pieces of gear I've ever owned and imo Grace's headphone amp design is as luscious and accurate as they get. But my new converter has a hi-end headphone amp built in, so time for someone else to enjoy this great piece of gear. Feel free to message me with any questions!

from Sweetwater.com:

Replacing the venerable m902, the Grace Designs m903 is a stereo reference headphone amplifier and monitoring controller housed in a sleek 1U half-rack enclosure. The m903 is designed to provide audio pros with an exceptional headphone amp along with stereo mains monitoring, whether from an analog or digital stereo source. The m903 accommodates these different sources with rear panel XLR and RCA jacks, and both +4dBu and -10dBV sources are easily managed. A pair of stereo TRS outputs allow two pairs of headphones to be driven by the m903. AES3, S/PDIF, and optical digital sources are accepted via rear-panel XLR, RCA and TOSlink connectors respectively. Plus, there's a USB port allows you to pump your favorite pleasure-listening tunes through the m903 for a true audiophile listening experience.

But the m903 doesn't stop there. Two pairs of line outputs (balanced and unbalanced) on the rear panel allow the m903 to act as a high-end monitor controller and system preamp. High-current transimpedance output amps can drive even the lowest impedance headphones, and a front panel rotary encoder provides precision level control of both headphone and line output levels, with level adjustments made in 0.5dB steps within a 95dB range.

Grace Designs m903 Headphone Amp/Monitor Controller Features at a Glance:

• Balanced and unbalanced analog inputs
• 24bit/192kHz digital stereo inputs- AES3, S/PDIF, TOSlink and USB
• High speed USB interface - completely asynchronous transfer mode allows for bit-perfect playback with zero interface induced jitter
• Driverless operation on Windows & Mac up to 96kHz/24bit
• Supports USB Class 2 audio specification for 192kHz/24bit operation on Windows & Mac (drivers needed for 192kHz operation under Windows)
• New generation DAC's and ultra low noise balanced current to voltage converter yields more musical performance compared to the m902
• Mono mode for checking mix phase issues (or listening to Pet Sounds)
• s-Lock phase lock loop sample clock regeneration for ultra-low jitter and rock solid digital stability
• High-current transimpedance headphone amplifier circuitry built to effortlessly drive low impedance headphones
• Balanced and unbalanced variable level analog line outputs
• Improved power supply architecture isolates headphone power amplifiers, line amplifiers, DAC's and digital logic
• Precision level control with a 99.5dB range in 0.5dB steps
• Channel level matching accuracy of 0.05dB
• Improved x-feed circuit eliminates perceived loss of low frequency content
• Optional infrared remote control available
• Internal linear power supply with custom wound toroidal transformer
• Sealed gold contact relays used for all signal switching