HiFiMAN EF5 - Hybrid Headphone Amp

This made my current headphones sound so much better, and did a wonderful job in powering them. This amplifier corrected some of the bass/treble inconsistencies in my headphones as well. This is one of the products I recommend most. The price point is very reasonable now.

Although I've only had this about a week, I've decided to post since there is a dearth of posted reviews.
First about me;
prior gear I have had - Grado Headphone amp, Headroom Micro amp, Stax SR-5 headphones, Grado 325 headphones
current gear -Music Hall 25.2 headphone amp, Grado 80 headphones, Hifiman HE-400 headphones
biases - tube gear, planar and electrostatic speakers.... headphones used when I feel too lazy to fire up the main system.
Audiophile purchaser since the 80's
 
Packaging when received was plain cardboard box with foam cut outs to house the product, no issues there with
shipping protection. The 12AU7 tube provided was a generic Chinese tube which I promptly put back in the box along
with the generic IEC cable. IMHO using these stock components is basically wasting the money you spent for a better amp.
IEC used was 1M PS Audio Jewel power cable. Out of the box I started with an RCA clear top 12AU7, more or less a pleasant
fail safe starting 12AU7 for all tube gear.
 
On initial play the amp presented good characteristics but  was a bit aggressive and overly dynamic. So after 30 minutes of sampling
I put JRMC on repeat play to break in the amp. After 24 hours of play time things seemed to settle in so I began to do more serious listening.
 
12AU7 Tube rolling- I have Golden Lion, Siemens, Funkwerk, Prima Luna , Telefunken (ribbed), Mullard CV4003, RCA clear top, Electro Harmonix
The Telefunkens as always are the cleanest sounding but expose a bit of an aggressive edge in the highs. The Mullard CV4003's damped
that edge at the loss of some finer detail.  I spent the majority of time listening using the Mullards but will report back if further break in eliminates
the edge with the Telefunken or I find a better matching tube.
 
Listening musts: like most tube gear the amp required full warm up for best performance. I found that 1 hour was best to get to maximum background
detail clarity. Cable performance differences also mirrored my main system so this hopefully means that if you have cables you already like, they will
work well also with the EF-5 and not cause a frenzy of new cable auditions.
 
The sound: the best acronym to use describing the EF-5 is "pleasing". It has tube mid range, clarity, along with solid state bite, and power.
With the HE-400's I typically listened at around 1o'clock on the volume knob, and did not have to  use the +10db dip switches on the bottom
of the unit. The Grado SR80's sounded nice also with the amp but sounded like the cheaper headphones they are, stressed by the demand of the additional
detail presented. This suggests that you want headphones with good power handling capabilities, lesser headphones with good source won't be pretty.
 
Comparison: compared to the 2 headphone amps I have had and the Music Hall I still have, this is a clear winner. The Grado comes closest for midrange magic
but the battery powered version had no oomph, couldn't convince on transients. I wasn't happy with the Music Hall or the Headroom Micro headphone amp but have no desire to bash them.
 
Areas for improvement: there are basically 2 things (1) the volume knob is very stiff and the position indicator hard to see. A dot of white paint would be nice and perhaps some tension reduction for easier turning (2)  low volume detail playback seems slightly compromised. My gut suspicion is that the OPA275 is the cause. I'd love it if HifiMan worked with Dexxa on a discrete op amp alternative that could work with their case fit limitation.

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.