i really want to embrace this earphone, the build quality is some of the best of all universals, earpiece is full metal and just feel solid. the loop is a good design for me. and if i don’t want to use the loop i can make it small and it work like a earbud. or take the wire out and wear it over the ear.
So i mainly listen to young female vocals, but the bass just muddy it up. male vocal sounds much better.
The bass is too overpowered, especially in the sub bass area, and the op bass extend all the way into the vocal band. The bass is slow and controlled, the quality is there, but too overpowered. I’d say that it is hard to even get this into my ear.
 

on phone I basically lower frequency below 500, boost 2k and 16k to my liking (poweramp)

it take some time to fit these properly
one of a kind sound
great mids and highs, female vocal is good, male vocal a bit thin.
At this price range, you can not get better bass, its the kind of not tight moving coil bass, the amount and the extension is out of the world. mids are not distorted due to 2 drivers config
good soundstage and transparency

got these as a replacement for an eol product. extra were returned
 
sound:
i don't know why bose made the remote versions, cause these don't really sound good when using smartphones, can't use them outdoor either
the open back design is my favorite, i just can't live with the enclosed sound of in-ears, isolation means loss of transparency, though there are a few sound isolating in-ear that offer open-back transparent sounds, but they are not as confortable. There is quite a lot of bass, but it dosen't distort the mid at all. and the bass is tight to certain extent, not balanced armature tight. rolled off highs means there is almost no sibilance in the vocals, and it sounds relaxed, I can listen to them for hours. the only down side is that the vocal can sound thin at times,
 
pretty good for $79

The playback portion of D100 has been optimized comparing to its predecessor D50. It can also playback flac and aac regardless of its inability to record in those format. It also feature multi language interface. Build quality is good, full metal construction. It also run on 4 AA batteries, and has good battery life, with back lit off, it can play for 24 hours straight.

Pros:
runs on AA batteries
dropping it isn't much of a problem
gapless playback for PCM and DSD
lag free interface, no file search time, no boot time
can drive most portable headphone well
 
Cons:
Heavy: 430g (0.95~ pound)
Channel imbalance at low volume, sensitive balance armatures is unlistenable (lowering the gain on music files or adding resistor cable can solve the problem)
attention magnet: better not have this out of the pocket when outside
 
Sound:
Overall, the D100 is slightly warm, very detailed. Instruments are better presented than vocal, sound is spacious and transparent, the soundstage is not too impressive however. The low on the D100 is a bit more than neutral, fast, punchy and extends deep. it makes instruments sounds very dynamic. The mid is detailed and full, the vocal will standout quite a bit, and this really shows when using V sounding headphones. The high seem overly extended when playing PCM files, it makes female vocals sounds great, lots of overtone, but can be tiring, and sibilance may be a problem for some people. in terms of highs DSD file sounds more relaxed.
 
Headphone choices: I really enjoyed paring the D100 with sr325 and er4s, I believe these two phones are great for vocals. the D100 is very forgiving for headphones choices,

Excuse my English
 
First off the DAC is 24bit no matter if its on PC/Mac/Linux
it Decodes PCM 192K and DSD 2.8/5.6 MHz
 
Build quality: One of the best on the market. the casing of the unit is 3mm thick aluminum. Components on the circuit board are neatly laid out.
 
Sound Quality: I don't have high end headphones, but Low impedance phones really sounds good (<150 ohm). The sound is balanced.
 
Filters: Fast and Slow roll-off for PCM, Low and High cut for DSD. PCM roll offs are easier to notice for the bass, DSD cut off is just lowering the ampitude as some DSD are recorded over 0db
 
There are also other features:
Optical input and output, can be used as a converter. the output is always on.
RCA output, i also have powered speaker that I use with the HP-A4.
 
Specs from fostex jp website:
 
Output power:
32 ohm load: 100mW
300 ohm load: 20mW
 
DAC chip: Taxas Instrument PCM1792A

Sound stage is average, but separation is awesome. Some people clam that bass is lacking, however, I think the sound signature is absolutely great.
 
Sounds good even when driven by laptop and phones. This headphone uses the same driver as 558 and 598 but the outer shell and ear pads aren’t as good, that is the only down side. But you can't expect too much for $80.
 
Edit: after 1 year of use i replaced the cushions with the 558 one (555/595), more confortable, and i have pop out the grill and replace it with a piece of very thin cloth.. so lightly modded, sounds bit more airy, if you still planning to get one of the 5 series should really jut buy the 598, pretty cheap now.. though now i prefer brighter headphones like grado 325. but I'd keep the 518 forever

The sound is one of a kind for BA, bass heavy sound signature. These are some of the bassiest BA you can find, with good instrument separation and soundstage. IMO really good for metal music, vocals are a bit far, v shape frequency responds.
 

Good phones, steady build, sounds ok out of the box. not bad for $100.
Updates
the headphone sounds really clean, with just the right amount of bass, maybe a bit less.

The sound isolation is really good with the sponge ear pieces, IMO better than a few of Sony’s active noise cancelling in ears (maybe active noise cancelling work better for low frequency e.g. during fight) I can hardly hear people talking right beside me. It’s also quite confortable. I won’t comment on the sound since it has dynamic drivers, out of the box it sounds average, give it some time and the sounds should improve quite a bit. Real good earphones!
 
Update:
i thought these are going to sound better after burn in, but it dosen't, still sounds bit dark.