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
Amazon.com: Bellari HA540 Pure Class A Stereo Headphone Amplifier : Electronics
McIntosh MHA200 Vacuum Tube Headphone Amplifier
Auris Audio Nirvana IV Tube Headphone Amplifier | HeadAmp
Amazon.com: XDUOO TA20 Plus Balanced Tube Headphone Amplifier Class A Amp Balanced in/Out Output Power up to 2000mW : Electronics