I've had my Emotiva Mini-X A-100 for a couple days now, so I'm going to share some impressions with the HE-400.
First of all, I had to figure out how to make an adapter. This amp has common ground, so the stock TRS jack of the Hifimans will work with an adapter. This kind of adapter will cause major problems with speaker amps that don't have common ground, so check with someone who knows what they are talking about if you try to make one like this for a different amp. For example, this won't work with a Topping T-amp.
My first attempt was to chop the female end off a 3.5 mm extension cable from Radioshack that I had laying around. I cut off the outer rubber layer, removed the flimsy foil shield, and stripped the ends of the L+ and R+ wires. The ground wire was bare, so i covered it with some of the outer layer that was left over. As you can see, it's pretty ghetto looking (not helped by that tape I put around it at the junction). However, it worked just fine. My biggest complaint was that the wires were very thin and fragile.
Note that you only need to connect one ground wire to either one of the negative terminals on the amp. L- and R- are connected inside the amp, and would be connected again at the sleeve of the TRS jack, so there is no need to have separate wires for them.
I put together a more solid adapter last night. I can't solder, so I used a Switchcraft model 1230 TRS female jack. It has screw terminals inside, so I just had to match up the terminals and wrap the wire ends around the screws and tighten them. This is very convenient and puts DIY adapters in the reach of anyone IMO. I used wire that I took from inside a Canare Starquad cable. In retrospect, it would have probably made more sense to order individual strands of wire than to take apart a cable, but whatever. Separating the individual wires means I can see which is which and don't have to do continuity testing with an ohmmeter. The other advantage of this adapter is that it's a 1/4" female, so I can use my 3.5mm to 1/4" adapter with 300 ohm resistors inside.
The Emotiva is obviously overkill for the HE-400. I'm ok with it. It's cheaper than almost every SS amp I can think of beyond the Magni and O2, which is why I got it. And because my Little Dot MKIII was clipping at high volumes, so I wanted MOAR power. MOOAARR!
Edited by manbear - 11/13/13 at 11:18am
Without the resistors, there was some noise floor. Not enough to ruin the experience for me, at about the level of ambient noise in my house. I could definitely hear it when things were dead silent in my listening room, but with the heating system running, washing machine on in the other room, etc. it was much less noticeable. With the resistors, the noise floor is totally gone on the HE-400. I can't even tell if the amp is on.
Each time I first used my two adapters, I plugged in my Nuforce IEMs before my HE-400s to make sure there weren't any shorts and that I matched the terminals correctly. Without the resistors, the noise on the IEMs was awful. With the resistors, there was about as much noise on the IEMS as there was with the HE-400 and no resistors.
Without the resistors, I had digital volume at around -10 to -20 db and the volume knob at 9 or 10 o'clock on the HE-400. With the resistors, I have digital volume at -0 db and the volume knob at 11 o'clock to noon, which is about what I expect from any amp. So the volume knob is very usable.
On to the sound compared to my Little Dot MKIII. The first thing I noticed was increased definition in the bass. Complex bass lines in electronic music became clearer (like this: http://www.youtube.com/watch?v=CP9r4g4qfbc), and the bass tightened while also gaining impact and snap. Extension was about the same. The sense of visceral texture in the midbass and lower midrange, the crunch and crackle of synths and guitars, improved noticeably. The Little Dot's bass sounded bloated and smeared after listening to the Emotiva.
Moving up the spectrum, the midrange became cleaner and faster. This amp really highlights the speed of the HE-400 IMO. The treble became a bit less smooth, but not offensive at all. Again, more speed and articulation. The Little Dot seemed to slightly blur the whole frequency spectrum in comparison. The Emotiva increased the size of the soundstage a bit, and increased separation and layering even more. On Daft Punk's RAM, for example, image placement became substantially more 3D. In general, I wouldn't say I'm noticing more details, but they are clearer and easier to pick out, especially sounds like synth layers in the background of mixes.
I also tried using the Little Dot as a preamp. It mostly sounded like I was still listening to the Little Dot, except there was obviously no clipping. It did however increase the usable range of the volume knob.