Pros: dynamics, drive, prace/rhythm/timing
Cons: bit "grayed out" in emotional tone and timbral color
So far I have used this amp with two different headphones: the Hifiman HE5LE, and the AKG K601.
I'll start with describing how it sounds with the AKG K601. First, let me note that one absolutely must put a 120 ohm series resistor at the output in order to be compatible with the K601. AKG specifies that the K601 should be driven by amp following the IEC standard, which is 120 ohms output impedance. And I can say that without that resistor, the highs are harsh and fatiguing. With that resistor, the sound is balanced.
The most outstanding feature of the EF5 is its dynamic power. Let me explain what that means. It does not mean simply that the amp gets loud. It has more to do with effortless transient power.
Before using this amp, I had no idea that I was dealing with compressed transients with all previous amps I have used. I didn't realize that I had just accepted compression in piano attacks, drum attacks, etc. But with the EF5, musical transients have this effortless quality, and arresting presence.
The imaging is superb. I know that some reviewers have claimed the headstage width of the EF5 is narrow. Well, I don't know what equipment they are using, but for me (using the K601, and also note that my source is a K Works SuperBerry DAC) the headstage is huge, and extends well outside the head. It feels very "open" in a quality that resembles the K1000 or HD800.
But more than huge headstage, the images are very precise. As an instrument starts playing, one senses that it has an exact location and a very attention-grabbing quality. To me, that ability to grab my attention by having a precise location in space is very important.
Now, about the Hifiman HE5LE headphone. This is an orthodynamic headphone, which means it is a planar-type. I must confess that I am not too enamoured with planar drivers. The HE5LE feels a little unnatural in its overall "feel" compared to the K601 (a dynamic-type headphone). The headstage is more compressed and closed-in. I think the dynamics are not as good as the K601.
However, it must be noted that Hifiman designed the EF5 specifically as a useful amp for driving the HE5LE. So one would expect they are compatible and one would expect to find merits in the combination. And yes, I do think the HE5LE sounds a little more balanced than the K601, and the highs are smoother.
Now about drawbacks to the EF5. Normally I use a SET tube amp, the DNA Sonett. The outstanding strength of SET's in general is the musical feeling, the timbral color, differentiation of moods and textures and so forth. And sure enough, the EF5 does not equal the DNA Sonett in these areas. It is a bit "grayed-out" in emotional tone and timbral color.
It is not surprising I like SET's because I generally prefer tubes over solid-state. The EF5 has a solid-state output. I am guessing this has something to do with its slight emotional grayness.
My intention is to the get the EF5 modded by K Works Audio. The guy there, Igor, has some tricks involving changing out the signal caps and putting in a better opamp. I am hoping that he can make the EF5 the equal of any SET in emotional feeling.