EDIT: For everyone who read this Feb 12 and Feb 13 2024, I screwed up. The cables I used, during one month, were faulty. Changing the speaker cables made a night and day difference. Review was re-written. No, SR-5’s don’t sound like “audiophile AM radio”. 
So, I wanted to try electrostatics/electrets/earspeakers for a while. I’ve been privileged enough to get the Stax SR-5’s (and the Stax SRD-6 energizer) for… a steal. They came with weird, old, flat, leather pads. They still sounded good with that, which is a feat in itself. But I had Shure HPAEC1540 pads lying around, and they fit perfectly on the SR-5’s. Comfort and sound vastly improved. They’re staying there.
Review was done with a Moon 100D DAC, “tubed” by a Little Dot Mk2, with the RCA out going to my Pioneer A-35R, which is a good old, detailed class A/B speaker amplifier (which is needed because, remember, this headphone cough earspeaker energizer needs to be plugged into speaker terminals).
Comparing to my Sennheiser HD6XX’s (plugged straight into my Little Dot Mk2), I’d say Stax SR-5’s are like, less punchy, less bass, more “air-y” HD6XX’s. Whereas the 6XX’s are known to give you great mids and naturalness, the Stax SR-5’s will instead give you detail with speed and precision, and everything, I mean everything will “jump” at you. Also… These are fast. Don’t expect anything to EVER sound muddy in these. Despite the apparent lack of bass and sub-bass (even compared to my Sennheiser 6XX’s), you still hear it. It’s still a well-made, gradual slope going down to 0 hz. They definitely won’t “punch” your earholes like Fostex T50RP planars. Everything will jump at you… respectfully (lol). And… not from “afar” either. There’s a lack of depth. i.e.: Even Gidge - Perimeter, which can make you feel like there’s speakers two meters behind the wall where your speakers are, won’t give you this impression of depth, even with these open-back “earspeakers”. Basically, SR-5’s got “eye-shaped” width and depth.
A bit of treble overcompensation?
There’s a bit of treble overcompensation, I believe. This is… what I expected, though, considering these are like 50 years old (yes – these were made in 1975). Most of the recordings were not as clear, nor bright, in 1975. Most of the speaker amplifiers had smooth, laid-back treble too. The thing is… I’m treble-sensitive. A lot. As in, I got rid of my Sennheiser HD58X’s because that was too much treble for me. But the treble in these SR-5’s is very well implemented. Addictive, even. If I want to remove the Stax SR-5’s from my ears, it’s not because there’s too much treble… it’s because there’s not enough of everything else. This is phenomental treble.
So, are these “old people headphones” ? Well, not really.
Old people being hard of hearing and losing the ability to hear the highest frequencies, you might say these are perfect “old people headphones”. Well… these are electrostatics. So, here’s an interesting result for you: Modern electronic music with “wall-of-sound” electronic drum kicks will sound fast, clear and modern with these (recommendation: Wragg and Log:One - Enemies of the Earth). Their speed and precision even makes em impressive for fast metal music with double-bass drums and blast beats. Stax can do a perfectly fast and detailed rendition of Testament - The Formation of Damnation. Yes. I’m headbanging to techno and thrash metal with these. This was definitely not what they had in mind in 1975.
Electrostatics: “I am speed”.
Hence… a comparison with my Fostex T50RP’s (planars). Planars tend to move faster than dynamic drivers. I already said “you don’t need coffee with these (planars)”. Well, my T50RP’s sound slow in comparison. Maybe I’m sensitive to the speed of the drivers (or maybe I’m just mentally ill), but using my T50RP’s can be stressful. But even though they punch multiple times as hard as the Stax SR-5’s… Well… it’s 3 AM. I’m writing this. I’m not asleep. These will wake up your brain. Their lack of sub-bass will also make you hear every little detail coming from the fastest drummers, without being “disturbed” by… Beats-like, overdone infrasonics.
TL,DR:
For every song that is NOT mid-oriented, the main problem I got with the Stax SR-5’s is the lack “life” in the mids. But otherwise, does it sound like I’m in the studio? Is Karen Souza and her deep voice in front of me? Well, pretty much. Even without EQ, John Butler Trio - Young and Wild, which got a lot of mids, was possibly the most life-like experience I ever had with headphones.
Also, my own system is meant to give me laid-back treble. Not because I hate treble, but because my ears are weird, and speakers/headphones with well-engineered treble and treble extension which won’t hurt my ears are hard to find, especially for cheap. These are the first headphones I own with addictive treble. The mistake I made (with the faulty cables), made me realize the potential of these to sound impressive. Someone already told me HD6XX had “godlike mids”. Get a treble-oriented system and Stax SR-5’s, in comparison, will give you “godlike treble”.
Considering these Stax are like 50 years old, even with the lack of depth (on my system), and sub-bass (which… is not much of a problem unless you mostly listen to electronic music), the result is mind blowing. These are great, all-around headphones, in which basically everything sounds either good or great. Hearing this is hearing the beginnings of surgical precision audiophilia. The ancestor of the “medical-grade” Audeze CRBN and all the other electrostatics… Except I got the Stax SR-5 earspeakers and the SRD-6 energizer for like 100$ (and yes, I immediately put 50$ Shure HPAEC1540 pads on em – and it’s 100% worth it).
For me, it all was worth it for the experience. It was worth it to try “estats”. It was worth it, if only to have a piece of audiophile history on my desk. But I guarantee you they’re more than that.
Pairing recommendations:
The simplest of recommendations would be to use these open-back headphones with a subwoofer (yes, some people do that… but, you know, your girlfriend, roommate or your cubicle co-workers might not like that, lol).
But above all, yes, if you can find an old school (or old-school-like, i.e.: 1975-like), mid-oriented Class A amplifier to go with the 1975’s Stax SR-5’s, go for it. These were made for these amps. For speaker amplifiers, don’t go for quantity, go for quality. i.e.: My Pioneer A35-R (35 watts per channel at 8 ohms) got more than enough power for the SRD-6 energizer (50% volume is too loud). There’s the Stax SRD-7 Mk2, which appears to be a better (well, the best) energizer (which you plug into speaker terminals) from Stax. Obviously, you can go the “SRM” route and get the legendary, huge ass 500$ Stax SRM-1 Mk2 amplifier + energizer… I’ll try not to, I already have enough amps I can use and try energizers with.
Follow up for more information about better pairings, because I’m not done.
I want a Stax SR-7 Mk2 (…with both normal bias and pro bias plugs, which would also mean I could try all the Stax made since 1985…), and I already wanted a pure Class A amplifier for my floorstanding speakers. I know these two things can possibly make my 50-year-old Stax SR-5’s the best headphones I heard. Pretty much the definition of a hidden gem, isn’t it?
EDIT: For your info, the wiring problems I had made me mess around and realize the SRD-6 energizer and SR-5 combo are incredibly sensitive to the wiring. Yes, the SRD-6 energizer’s red+black+green+white wires which go straight to the amp. Adding 5 foot to this cable will literally remove 5 or even 10 decibels of bass and sub-bass from the SR-5’s. So if your SR-5’s lack bass and sub-bass and the 4 feet long SRD-6 energizer’s red+black+green+white wires are too long for you, simply cutting 2 feet of it might just be what you need.
…note I’m talking about the amp-to-energizer cable, which is simply speaker cable. I’m not talking about the headphone-to-energizer cable which is apparently 280 volts I’m and not responsible if you temper with this 50-year-old cable and electrocute yourself and/or die etc., and for this reason I won’t ever mess with it myself either (for me the SR-5’s are also, as I said, “a piece of audiophile history” – better left as is).