The NAD M51 DAC was on Stereophile's Recommended Components list with an A+ rating, a distinction it shared with DAC costing multiples more. It is in perfect condition. I have the original box and the remote. You can use its internal preamp or set it up to use the preamp of your choice (this is the way I have it set up now).
Description: Remote-controlled digital-to-analog converter. Digital inputs: AES/EBU (XLR), S/PDIF coaxial & optical, 2 HDMI (stereo only), USB 2.0; all support up to 24-bit/192kHz. Digital output: HDMI. Analog outputs: left & right RCA (unbalanced), left & right XLR (balanced). Output impedance: 47 ohms (unbalanced), 110 ohms (balanced). Output level: 2V RMS. Frequency response: ±0.5dB (ref. 20Hz–96kHz at 192kHz sample rate. Signal/noise: >125dB (ref. 0dBFS 2V out). Channel separation: >115dB (ref. 0dBFS volume –1dB). AC supply: 100–240V, 50–60Hz. Power requirement: 16W.
Dimensions: 17" (435mm) W by 3" (78mm) H by 11.5" (296mm) D. Weight: 12.8 lbs (5.8kg).