The Rega EAR (I use the MKII version) design is thought out with high impedance headphones in mind. I own the Sennheiser HD 650 and I've owned the HiFiMAN HE-400i and I couldn't agree mor with Amir's opinion.
If you own a Sennheiser HD 650, the Rega EAR is a great match that will make them shine. I feed my Mark II with the output from an iFi iDSD nano that I use as a DAC connected to my computer and the sound is fantastic.
I have two complete sets in different locations, one with a Windows 10 Pro computer as source where I use MusicBee as my FLAC library player, and another one connected to an iMac with Audirvana, and both are great.
My only concerns with the Rega EAR Mark II is that construction quality is sub-par and also that I can only use less than a quarter of the volume control (in fact from 18:00 to 20:00).
For me is a perfect choice with the Sennheiser HD 650, and I've tried a lot of amps at home, including some respected names like the Burson Conductor or Icon Audio HP8 MK II.