Little know factoid, the headphone amp from the Benchmark DAC-1 is available as a separate component, but you must do some construction work!
specs:
Input Stereo 200 kΩ Balanced Inputs
Input Clip +26.5 dBu with ±15 volt supplies
Output Stereo, for use with 1/4" Tip/Ring/Sleeve plugs only, other panel connectors may be wired to the amplifier by way of on-board output connectors
Output Z "0 Ω"
Output Clip Dependant upon power voltage and load [+20 dBu @ ±15V and 60 Ω load]
Load Z 30 . or higher
Output Level Up to +20 dBu into 60 Ω [dependent upon power source]
Output Control Dual log taper response
Gain Range Off to +20 dB
Bandwidth 0.1 Hz to 300 kHz minimum
THD+N 0.0003% typical at headphone inputs - under load!
Noise Floor Better than -75 dBu at maximum gain
Power (Bipolar) ± 9 to ± 18 volts, maximum peak current needed = 1.1 Amp at maximum peak output into 30 Ω
Peak Output 7.5 watts [±18 volt supplies. 30 Ω load]
Max Avg Out 1.35 watts [-8 dB peak to average ratio]
Amplifier Board length behind panel = 3.760", width = 1.335", mounting hole diameter = 0.437" [7/16"]
Gain Ctrl Board length behind panel = 1.025", width = 1.335", mounting hole diameter = 0.275" [J drill or 9/32 (0.281)], shaft length = 0.385", shaft diameter. = 0.235" [16-star spline]
Spacing Minimum spacing between Gain Control Board mounting hole and 1/4" jack = 7.5" Minimum spacing between Amplifier Board and Gain Control Board = 0.75" [both boards mounted vertically], Maximum panel thickness = 0.1". Knob diameter = 0.75", length = 0.75"
Knob Volume control knob included
My notes:If you give it good, very clean +/- 15vdc and a well-shielded box, it will drive a pair of Senn 600's into uber-happyland