I use the S.M.S.L. sApII Pro to drive 300Ohm Sennheiser HD600s and it performs really well. It delivers plenty of clean, flat power to drive my headphones with authority, just what a headphone amplifier should do in my opinion. There is not much information around but I would say the amp is designed for high impedance headphones.  I do not recommend getting this amplifier to use with low impedance headphones as there is channel imbalance at low volume. I can hear this while listening to IEMs at lowest audible volume.
 
The amp comes with a source switch which allows you to hook up two sources and switch between the RCA and 3.5mm input, nice! Negatives would be that the volume pot makes some noise and that the amp does get warm after a while.
 
In a world of overpriced HiFi gear I believe this is a bargain at around 70 bucks.

Overview
At a retail price of $119 the 9018D has a lot going for it. The high resolution Sabre ES9018 chip coupled with an asynchronous USB receiver provides a crystal-clear sound and is a significant upgrade compared to the sound card of my MacBook Pro. As an owner of the regular 9018 the most apparent upgrades of the 9018D are a higher max volume output, digital volume control and the aluminum casing. As a company previously known for DIY products HiFime has stepped up their game with this DAC. Forget about plastics and enjoy the high quality feel & finish of the 9018D.

Sleek looking 9018D
 
Using the 9018D 
On Mac OS X the unit is plug-and-play. No drivers, no hassle. The slightly backlit display shows the sample rate in a subtle way and lights up when using the volume knobs to clearly display the volume setting. The provided USB cable is of high quality. I do miss bumpons to protect the DAC against scratches. It would be nice to include these so the owner can decide to install them or not (as with the Fiio E10k for instance).
 
Update (April 19): I just found out that you can adjust the screen brightness and PCM filter frequency response by pressing the volume knobs during start up. Really cool to have these features which enable you to customize the DAC to personal preferences.
 
Sound signature and pairing
My setup:
mix of FLAC/DSD/TIDAL/Spotify > 9018D > SMSL sAp II Pro > Beyers DT990 250Ohm/Sennheiser HD600
 
The sound produced by this DAC is (micro)detailed with great clarity. It delivers music with a superb air and with the right amount of space between the instruments.  Overall I did found it to be slightly bright which I believe Sabre chips are generally known for.
 
I did not find it a good match with my Beyerdynamic DT990 Pro 250 Ohm. The slightly bright 9018D did not complement the “V-shaped sound” of the Beyers. Please note that I am very treble sensitive and the DT990’s treble did not suit me very well to start with. After some time I decided to exchange my Beyers for a pair of Sennheiser HD600s. Now this is a perfect match! The HD600, which is considered to have a somewhat “veiled” sound, is complemented very well with the detailed and slightly bright 9018D. I would also consider this to be a good match with the darker Sennheiser HD650.
 
The volume goes loud enough to drive high impedance heaphones but the output power is not sufficient to reach the full potential of 300 Ohm headphones. Connected to a proper amp the soundstage will significantly increase and according to me the 9018D is still not a perfect all-in-one solution for high impedance headphones.

Match made in heaven
 
Volume control
First off all I would like to report that volume is controlled in two ways with this DAC. The system volume will control the volume at the USB receiver’s level and works in 8 steps only (behaviour of SA9227 USB receiver). Secondly you can adjust volume with the build-in 255 step digital volume control (0 to -127.5db in 0,5db steps). This wide range ensures that the DAC can be used with anything ranging from sensitive IEMs up to high impedance headphones.
 
Digital volume control is generally regarded to be inferior to an analog volume control. In the case of the 9018D the digital volume control has access to the DAC’s datapath and this removes noise associated with digital volume controls (more info here). It works perfectly and you can actually hear a tiny amount of noise if you change volume when no music is playing (only with sensitive IEMs). As soon as you then play music and hit pause the noise is completely gone.

Display while using volume knobs
 
DSD support
DSD files sound really good. Overall you get a smoother more relaxed sound with even more micro details and great resolution. If you want to play DSD files the system volume must be set at 100% and listening volume controlled with the digital volume control. This is because the system volume will alter the signal before going into the DAC chip. With any setting below 100% playback of DSD files is impossible and instead you will hear hiss. The system volume should also be kept at 100% to ensure the full dynamic range of your recordings.
 
So basically keep the system volume at max and adjust the volume with the digital volume control. Or if coupled to an amp you can adjust the volume on your amp. In my setup the 9018D’s digital volume is fixed at -25db to stay below the maximum input voltage of my amp on which I set the actual listening volume.
 
Comparison
The 9018 and 9018D sound identical to me. I would say the big difference between the two is heat dissipation. The aluminum construction of the 9018 dissipates heat much better than the tiny plastic 9018. You also cannot adjust volume on the 9018 during DSD playback for reasons explained above as it only has the volume control at USB level. This means that with headphones connected directly to the 9018 you are stuck at the highest volume level which might be a deal breaker for some.

Tiny vs small
 
Compared to the Fiio E10k the 9018D sounds more detailed, airy and has a wider/deeper soundstage. The Fiio sounds warmer and seems to have a slightly better amp. The choice between the two will depend on your sonic preferences. The E10k does not warm up in use while the 9018D can get fairly warm, if you mind that sorta thing... 

Two great value DACs
 
Conclusion
The Hifime 9018D is a budget friendly and capable (AMP/)DAC. It sounds great, has an attractive design and proper digital volume control. The capability to play high resolution and DSD files is remarkable at this pricepoint. It can be used with sensitive IEMs up to high impedance headphones, although the latter still benefit from a more powerful amp. If you are going to use your DAC to feed an amp with volume controI I suggest looking into the regular HiFime 9018. As a standalone DAC which supports DSD playback the 9018D can’t be beaten! 

Overview

The HiFime 9018 is a definite improvement compared to the sound card of my 2014 Macbook Pro. It sounds very natural and detailed. I would still recommend an amp to drive high impedance headphones like the HD650 (mentioned in the HiFime product description) and my pair of DT990 250 Ohm though… The DAC will drive them to a loud listening volume but nothing more. HiFime informed me that in order to make the DAC work with a variety of headphones they limited the output power as the Savitech USB receiver only has a 8 step volume control. The upcoming 9018D with its lossless digital 255 step volume control has a higher max output power and can be used for a greater range of headphones (sensitive IEMs till high impedance HPS).

 
Comparison
With their similar price tags I compared the HiFime9018 to the Fiio E10k. I think they are really close with an edge towards the 9018s sound. Then again the Fiio has a more powerful amp stage and high quality finished housing. The Sabre ES9018K2M 32bit DAC chip is nice spec-wise but honestly I do not hear improvements in audio quality above 44,1kbps/16bits PCM files. DSD is another story...
 

DSD support
Playing native DSD128 stereo files in Audirvana Plus is where the HiFime 9018 really shines. I was really impressed with my first DSD128 experience and I think it has an edge over high-end FLACs.
While being impressed my biggest disappointment was with the DSD playback as well. The volume control is done by the USB receiver and this means that the signal is altered before going into the DAC chip and thus playback of DSD is impossible below 0db (highest volume level). If you use this DAC to feed an amp you're fine but when connected directly to headphones you're stuck at the highest volume level. Luckily for me this still is a comfortable level with the DT990 250 Ohm. The 9018D which I mentioned earlier also addresses this issue as the digital volume control has access to the DAC data path (explanation here).
 
Nice to know
"One more thing" which I find really nice is the tiny footprint and light weight. The housing does get warm due to the packed double PCB but nothing I am worried about.
The plastics give the unit a somewhat plain look and the finish is below level compared to others in its price range (read Fiio E10k). This being said it does feel very solid.

 
Native android 5.0 support works like a charm on my Nexus 7 with an OTG cable, really nice for those bedtime listening sessions .
 
The folks of HiFime are really nice and try to answer any questions you might have, thumbs-up!
 
Summary
The HiFime 9018 delivers! The Sabre ES9018 chip, usually found in higher priced DACs, produces a natural detailed sound which will leave you smiling while listening to your favorite tunes.
If you are looking for the best bang for buck USB DSD DAC to connect to an amp look no further. As a standalone DAC/headphone amp there are limitations for high impedance headphones (+200 Ohm) and DSD playback, in this case my recommendation would be to wait for the HiFime 9018D USB DAC (preorder). If you don't care about DSD playback ( you should ) and have relatively low impedance headphones you could go for the HiFimeDIY Sabre U2 Asynchronous USB DAC at $57.00.