There is something machiavellian about HIFIMAN putting out a desktop DAC/amp which works best with its own headphones and barely so with others on the market. I suppose the writing was on the cards when the EF400 launched alongside a loyalty discount, whereby those who had purchased other HIFIMAN headphones recently could avail the EF400 for a highly attractive price. In this regard, I can't blame HIFIMAN for aiming to appease its customers by releasing a new source which works so well with the HIFIMAN Arya especially and the Ananda/HE1000se/Edition XS/Susvara to varying other degrees of success, that I can certainly recommend it to those who fall in this category. In fact, I'll expand it slightly to include most detailed planar magnetic headphones that go with a neutral-bright tonality—HIFIMAN just happens to be the de-facto representative for this sound signature. It can also work well for those who want a warm smoothening filter over the sound from other headphones but that's at a sacrifice of accuracy and tonal balance that I can't get behind more often than not.

The EF400 uses HIFIMAN's self-developed "HIMALAYA" R-2R DAC topology paired with a Class AB buffer and voltage + current amplification modules to where it's about as far from your typical ESS/AKM DAC and op-amps source as could be. There are quite a few variables here which affect the overall sound signature even with the higher output impedances that make it less amenable for use with many IEMs. The DAC itself is one of the few occasions where it results in a tangible difference to the sound—generally warmer and less clean while also having synergy with some products far more than others. HIFIMAN also provides two gain settings and two operating states of oversampling or not to where I found a specific combination of high gain, NOS, and balanced headphones outputs to work best for me. In addition to these more goldilock's style conditions of optimal behavior, note that the single USB input is also somewhat kept back by the bridge interface to where you won't have all the playback rates possible on less expensive gear. The HIFIMAN EF400 ends up being a source that is hard to recommend for everyone thus, and yet almost feels like a must-have for some! In the right conditions I absolutely adored it enough to where I would urge you to at least give it a shot at an audio show/dealership, and it does cost a reasonable amount if you wish to indulge in a source that is far removed from the usual formula most others are operating in.