Author's Professional Headphones and Amplifier 16-600 Ohms 20Hz-20KHz class A | eBay
Amazon.com: beyerdynamic DT 990 Edition 600 Ohm Over-Ear-Stereo Headphones. Open design, wired, high-end for use with headphone amplifiers : Electronics
Amazon.com: FX-Audio Tube-02 Pro Tube Headphone Amplifier, Vacuum 5725 Tube Preamp, Mini Hi-Fi Integrated Stereo Class A amp Supports 32-600 ohms of Headphones, for Stereo Amplifier/Active Speaker : Electronics
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