Massdrop Objective 2 (O2) Headphone Amplifier $79.99 (or upgraded version: $94.99) + shipping
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This is a highly rated headphone amp (no DAC) which is normally quite a bit more when built by other companies. I highly recommend getting the desktop version for 15$ more (same amp is $167 for the jdslabs built one). Would work great paired with a 6XX (or pretty much any set of headphones).
Baseline Perfect
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.
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