Stax, SR-5 with SRD-6 Adapter - Reviews
Pros: Ultra sharp transients give great attacks, transparency, pitch clarity (even in bass), and dead-real drums; comfortable.
Cons: Light bass, restricted sound stage, requires wall plug power and speaker output to drive, hence for home system.
I had by happenstance acquired a pair of these in the mid-1970s, well before my interest in quality headphones, and then set them aside in a box of "junk" electronics, where they have remained for nearly 40 years, since they didn't fit into the headphone jack of any of my systems.  When I saw a post recently that sung the praises of these, I had to dust them off, run appropriate wires for my home system, and plug them in.
I could not put them down and stayed up until 3:00 am trying various tracks through these headphones.  I had purchased a pair of Grado PS500s within the past few weeks as my best pair of headphones so far, but my initial listen made me think that these were better (more detail on that statement below!).
Sound Quality
In short, these headphones are "bright," but not "in-your-face" bright.  The attack of a drum stick on a snare drum comes across just as if it were live, suggesting great transient response.  There is no "veil" between you and the instruments... regardless of pitch or timbre.  I would imagine one would call these headphones "fast."
Lower bass is light, but not unpleasantly so.  In fact, I can discern the pitch of each note in a complicated plucked bass sequence in the face of other instruments, as the clarity and lack of fuzzy boominess in the bass more than compensates for its lesser relative volume.
I have been comparing headphones, in sets of three, in a rather systematic manner, so I imposed those tests on these to provide some comparisons.  I compared these Stax SR-5 headphones with SRD-6 drivers to both my best headphones (Grado PS500) and my best IEMs (Klipsch S4i), with the idea being that IEMs might come closer to the "fast" response I was hearing from the Stax than the Grado.
To test various aspects of sound quality, I used an iPod Touch 5th Gen source patched into my Technics full-sized stereo system.  The SRD-6 was driven by the "remote" speaker output and in turn drove the SR-5 headphones.  I devised a set of 10 comparative tests (4 that are of an overall quality nature that do not depend a lot on the music being played; 6 that compare the clarity of specific acoustic "events" in certain music). I describe the test methods more fully here .
Rather than trying to give an absolute score to each headphone for each criterion, I simply rank ordered them, based on back-and-forth pairwise listening for each test and each pair of the three headphones (took between and hour and an hour and a half).
The overall, or "macro tests," were briefly (more detail in post cited above):
  1. Transparency;
  2. Size, both horizontal and vertical, of sound stage;
  3. Resolution of position of two persons singing near each other;
  4. Volume of headphone with iPod turned up all the way.
The event-based tests were:
  1. "Twang" of drumhead at entrance to Song 1;
  2. Preservation of features allowing me to determine pitch of bass notes in Song1 Verse 3)
  3. Finger pluck at start of bass notes at start of Song 2;
  4. Clarity of shaker, preserving differences of each shake, in Song 2 Verse 3;
  5. "Ripping" sound characteristic of horns and medium low reed organ pipes at start of Song 3;
  6. Ability to hear additional echoing chord stacked upon a huge bombast of sustained full orchestra and organ four beats later, in about third "verse" of Song 3.
Here is the result of my comparison.  A 3 indicates that headphone was the best of the three in that test and contributes 3 points to an eventual headphone score totaled at the end... a 1 means it was the worst.
TestGrado PS500Stax SR-5 w/ SRD-6Klipsch S4i (IEM)
Transparency123 (best)
Width of sound st321
Positional resolution321
Drum "twang"231
Bass pitch perception321
Bass finger pluck123
Shaker variation123
"Ripping" of organ/brass231
Discern added chord23 (tie)3 (tie)
Since these scores reflect rank order position rather than absolute ability, total scores can be deceptive.  For example, the width of sound stage of both the Grado and the Stax were very similar, with the Grado doing just a bit better, while the Klipsch soundstage was so reduced as to render the whole IEM a bit disappointing... yet that only amounts a difference of one or two points on one of 10 criteria.  So even though the Grado and the Klipsch score equally, the Grado is far preferable.
More preferable still is the Stax, by a significant amount.  None of these tests really capture the relative lightness of its bass.  Published plots of frequency response show a flat contour down to about 60 Hz, from which point the response drops off by about 8 dB as one moves down to 20 Hz (marketing parlance would still call this "flat +/-4 dB.")  However, non-sound attributes limit its attractiveness:
Quality Excluding Sound:
The Stax requires both a wall plug for power and a speaker output for signal.  Hence, it requires that the listener come to it, rather than allowing the listener to take it along, as wall plugs require walls and speakers usually require large floor- or rack-mounted stereo systems.  The SRD-6 transformer box provides a switch and set of speaker outputs, so one can use it to switch between sending the music to full size speakers or to the Stax ear speakers.
The headphone is very comfortable.  Its over-ear design make it more acceptable for long listening sessions than the Grado PS500, which is on ear and can become fatiguing as it presses your ears against your head.
I perceive no degradation to the Stax from sitting for years and Years and YEARS in a junk box of electronics with tangled cables and sharp-cornered small aluminum electronics boxes (headphone switches and the like).  Both side are balanced (some writers caution against channel imbalance for Stax). 
I believe that the Stax SR-5 ear speaker and its SRD-6 transformer constitute one of the least expensive ways of leapfrogging into audiophile quality on a budget.  The system is often offered on eBay for a few hundred dollars.
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Reactions: nick n and Savant
Kayakjohn, what is the difference between this Stax and the Stax SR-5N? Is the 5N higher value?