This is a highly rated headphone amp (no DAC) which is normally quite a bit more when built by other companies. I highly recommend getting the desktop version for 15$ more (same amp is $167 for the jdslabs built one). Would work great paired with a 6XX (or pretty much any set of headphones).
In 2011, audio engineer and blogger NwAvGuy published a post titled "O2 Headphone Amp." In the blog, he outlined his philosophy on defining sound quality and published his ideal audiophile circuit design. This is the Objective 2, NwAvGuy's most famous headphone amplifier. Capable of powering everything from IEMs to HD800s with linear ease, the O2 is your ideal baseline reference amplifier.
Note: At checkout, you'll have the option to choose between the Standard Edition and the Desktop Edition (+ $15). The Desktop Edition features a quarter-inch headphone output, RCA inputs, and rear mounted power jack. For the Standard and Desktop Editions, you can select either the 110V or 220V AC Adapter, and select the Standard Gain (2.5x Low, 6.5x High) or Medium Gain (1.0x Low, 3.3x High).
Input: 3.5mm Stereo Connector (Standard Edition), RCA (Desktop Edition)
Output: 3.5 mm (Standard Edition), 6.35mm (Desktop Edition)
Power Battery (not included) or AC Adapter (included, choose 110v or 220v at checkout)
Gain: 2.5x and 6.5x (or 1.0x and 3.3x, select your configuration at checkout)
Volume potentiometer taper: Alps 15A or 3B
Frequency response: +/-0.1 dB (20Hz-20kHz)
THD 1 Khz 150 Ohms: 0.0016%
IMD CCIF 15 Ohms: 0.001%
IMD SMPTE: 0.002%
Noise (ref 400 mV): -105 dB
Max output (33 Ohms): 613 mW
Max output (150 Ohms): 355 mW
Max output (600 Ohms): 88 mW
Output impedance: 0.54 ohms
Crosstalk (15 ohms): -65 dB
Channel Balance (50% volume): 0.6 dB
Case dimensions: 4.12 x 3.87 x 1.12 in (10.46 x 9.82 x 2.85 cm)
Weight: 9 oz
O2 Headphone Amplifier
110V or 220V AC Adapter