I had a lot of trouble writing this review, but not for the usual reasons. I wasn't ambivalent, nor did I find myself struggling for words to describe my experience. It was that I knew that as soon as this posts, I'll have to put these back in the box they came in, and ship them back off to Fostex.
Fit and Form
The headphones' ear-cups are made of a beautiful Japanese Cherry Birch, with an "Urushi" lacquer, which is applied by a 110-years old group of artisans by hand in Japan. Linked below is a video which details the intensive process that each ear-cup is put through before it reaches the production line; I highly recommend it for anyone with ~5 minutes to kill and an appreciation for how much work goes into just the ear-cups of these headphones. I can't overstate how, in person, these ear-cups are probably the prettiest thing that I've seen on a headphone to date.
Moving around the headphones from there, the ear-cups are attached with a relatively unique and (in my opinion) very solidly-designed hinge that allows them to rotate, although not fully. From there, there is the size adjustment, which clicks resoundingly, and hasn't moved untowardly in my usage. The headband is wrapped in a synthetic leather.
The pads on the ear-cups are attached to a plastic ring with rotates to lock, making switching pads very easy. I didn't do any pad-rolling while I've had these, but it is something to consider for prospective buyers, and counts as a plus in my book.
Having broken down the components, I'd like to very generally comment upon comfort. I think this was my biggest qualm with the headphone, but it certainly isn't bad by any means. I came from reviewing Cascadia Audio's Talos, one of, if not, the most comfortable headphones I've ever worn. Comparatively, the TH-900 Mk II rest with the majority of their weight directly down on the head, and the rest being pushed against the insertion of the user's jaw. The ear-pads don't quite evenly distribute pressure on the area surrounding the outer ear. Now they don't weigh much, so this wasn't a huge issue, but I felt that, given how tremendous they sound (which I'll get to soon), Fostex could have dedicated a little bit more development to the headphone's comfort.
Completely separate from all this, although still relevant: there is, included, a Fostex branded headphone stand. I personally appreciated this, as it gave them a dedicated place on my desk, always in reach. I saw this as a nice touch that Fostex was not obligated to include, but decided to. Similarly, in the accessories category, is a synthetic leather carrying bag, although I can't imagine many buyers carrying these around for portable use. But hey, to each their own, and it certainly is nice to have in case.
Finally in this section is the cable. It is 3 meters long and fabric-wrapped. Interestingly (for a pair of over-ear headphones), I found it to be somewhat microphonic, but I only noticed it when they were sitting on my head without music playing, so that wasn't an issue in use. The cable inserts at the ear-cup (on both sides) with a two-pin connector that I have not seen before. This connector only inserts one way, but it is marked, so that wasn't too much trouble. At the other end is a 1/4" TRS auxiliary connector, which felt very solid and premium; it's subtly branded with a Fostex logo and has a sleeve at the end further from insertion for stress-relief.
Now that all that is out of the way, let's get to what matters most: sound. This headphones offers a slightly bass-elevated sound signature with the single most musical implementation that I've ever heard. The 50mm biodynamic driver certainly helps here, as it offers a velocity of response like an aluminum driver, with the sound of a paper driver.
For anyone who has gotten a little bored of their music collection and has the money to burn, I recommend these, very strongly. They imbued a vitality to almost every single recording I put through them, from the most clinical to the most fluid music that I could think of, in very gestural terms. They fall short no where, with vocals of neither gender ever receding or becoming distant. I'll break down what I mean by that in the following sections.
The soundstage is surprisingly large for a pair of closed-back headphones. It doesn't rival open-back designs, but it certainly is impressive given the form of the headphone. Imaging and separation are top notch, with each and every instrument coming across as wholly distinct, regardless of how dense the mix is.
Songs used: I Never Woke Up in Handcuffs Before by Hans Zimmer, Temptation by Joey Bada$$, and America feat. Black Thought, Chuck D, Big Lenbo, & No I.D. by Logic
Although I sincerely haven't found a low point in these headphones' sound, I think the bass is certainly a, if not the, high point. It is absolutely fulfilling, but without losing a single bit of definition. It doesn't bleed into the mids, or become muddy or sloppy in the slightest. While simultaneously being musical and rounded, it renders every single detail with full fidelity. I was particularly impressed with its rendering of "I Never Woke Up in Handcuffs Before" by Hans Zimmer, and many headphones have issues with both the speed of the bass in some passages, but also with portraying the rounded nature of the bass drum naturally and realistically. The TH-900 didn't stumble here, nor anywhere else in my testing with this frequency range. When it's called for, it can provide incredible slam, but can also deftly handle intricate and layered passages without a single misstep.
Key descriptors: fulfilling, ensconcing, and captivating
Songs used: Goon Squad by Ripe, Cream on Chrome by Ratatat, and Black Mambo by Glass Animals
The mids on this headphone are almost as laudable as the bass. They are creamy and rich, while still remaining detailed. I had a particularly hard time qualifying the mids, as they can be whatever the song calls for. In dense, precisely-articulated instrumental passages, there is no blur between the lines, everything is distinct. In soupier passages of other songs, they come through as lush and satiating in a really unique and special way. I enjoyed solo piano pieces particularly as there was a real sense of space and precision to the music through these headphones.
Key descriptors: lush, precise, and dense
Songs used: 22 (OVER S∞∞N) by Bon Iver, Ageispolis by Aphex Twin, and this triangle solo by Jose Cortijo
Please excuse the linked Youtube clip, but I found that that particular solo does a spectacular job of putting a headphone through its paces in terms of sibilance. Although pointed and poignant, I didn't experience any sibilance with this track, which I did not expect. Aside from that specificity, the treble is airy and precise, with great extension and detail in every regard. This headphone really does not sacrifice anything in terms of sound, and it really, really impressed me.
Key descriptors: precise, detailed, and airy
Minutia and Miscellaneous
Aside from the comfort, I didn't find anything that particularly stuck out to me as wrong with this headphone, and even that wasn't wrong, per se. I just felt that Fostex could have spent a little bit more time working on the fit of this headphone, along with its absolutely incredible sound.
Here's my tell-all: if you have the cash, and what I've said in the above review entices you, absolutely buy this headphone, given that you can find it in stock. Reading back through my written impressions of it, I think that I've done the best I can to put my experience into words, but it still falls short of accurately capturing everything that I appreciate about this headphone. It is utterly cohesive and beautiful in pretty much every regard. For final words, it breathed life back into recordings that I had long lost interest in, and did so effortlessly.
These headphones were provided to me for review by Fostex. I am not being paid by anyone to write this review, to endorse the product reviewed, or for the content that I put in the review.
These headphones were powered by an Astell&Kern AK Junior, a Google Pixel, a Neurochrome HP-1, an iFi Pro iCan, and by whatever soundcard is in the motherboard of my computer.
I have had these headphones for about a month, and I have put about 85 hours of analytical listening through them during that period.