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
Portable Amp Diy Amp Hifi Headphone Amplifier With Dac Chip Can Push 600 Ohm Headphones - Transformers - AliExpress
Amazon.com: beyerdynamic DT 990 Edition 600 Ohm Over-Ear-Stereo Headphones. Open design, wired, high-end for use with headphone amplifiers : Electronics