Seems like the search function is invisible to most people. I've ranted on and on about how the SR-001 compares to IEMs and full-size cans, including, yes the HD600 and HD650.
But what the heck, I've got time to kill...
The SR-001 will soundly beat most IEMs in terms of sound quality. I have not heard a single universal-fit, except the E500 when it is driven by a synergistic amp (in my case the original Hornet), that's anywhere near in overall sound quality. In terms of custom IEMs, the ES2 is, frankly, better, but it is also one heck of a lot more expensive. The ES2 is better because it is much more well-rounded, whereas the SR-001 has certain very visible sonic flaws, but what it does well, it does much better than the ES2.
The sound signature is warm, with a lot of lower midrange emphasis specifically and a lot of midrange emphasis in general. The treble is rolled-off and not very extended. The bass is fairly linear, with some midbass emphasis but less than many dynamic phones. Together, it makes for a colored but very seductive sound. Vocals and guitars readily stand out, as does anything that's solely in the midrange really, while the bass is punchy and authoritative for an electrostat, but not quite as deep as it could be. Because the treble is recessed, and because electrostats tend to sound diffuse, textures don't come out nearly as crisp as they could be, so everything has a very ethereal sound which is quite beautiful but not very realistic. The soundstage varies from very small to fairly large depending on associated equipment. Imaging is fairly accurate but instrument separation is not as good as other stats. Images will bleed into each other and instruments seem to overlap in space as opposed to being very clearly separated. There is also an airy haze around a lot of instruments when the headphone is used with tube gear (I use the SR-003 with the SRM-007t, for instance) but when you're using the SR-001 out of an mp3 player I doubt you'll have to worry about it that much.
This headphone does very well with rock, jazz, blues, and anything with a lot of midrange emphasis. It doesn't do so well with electronica because of the recessed treble, and doesn't do quite as well with classical because of the smallish soundstage, colored sound and generally hazy instrument separation. However, by "not so well" I mean that it doesn't so as well as the genres that suit it. On the whole, it's still fairly competent.
Naturally, this is an electrostatic driver, so it will have the sonic characteristics of one. With a dynamic driver, you have a lot of excursion, and the driver is capable of moving a lot of air. That means that you get serious bass punch (usually), and a palpable, solid sensation to the sound (this is very present on the HD600, for instance). However, the diaphragm also has a heavy voice coil hanging off it, which means that it cannot respond to changes in the signal very quickly, which in turn hurts transient response and overall speed. So, when the music gets complex, dynamics start to lose resolution and tend to mush everything together (the HD600 is very guilty of this).
Electrostatic drivers, on the other hand, don't have a voice coil hanging off the membrane, so the membrane is, for all intents and purposes, weightless. That means serious fast transient response and virtually no distortion whatsoever. This also means that as the music gets more and more complex, electrostats start to perform better and better and pull way ahead of dynamics. Even when you torture a 'stat with something as abysmally fast and complicated as, let's say, Nile's "Black Seeds of Vengeance" (brutal progressive death metal) or Stravinsky's "Petrushka" (one of the most complex orchestral pieces I've ever heard) you can still hear the texture and tone of every individual instrument. A dynamic here would lose sight of the microdetails and give you a more general, blurred sonic picture.
On the downside, an electrostatic driver is capable of very limited excursion. That means that it cannot move nearly as much air as a dynamic, so you're lacking the tactile, palpable sensation that a dynamic driver produces. You hear the bass, but you don't feel the slam (as much, you still feel a bit of slam). The treble shouts but it doesn't bite, the drums aren't as real and solid, and everything sounds more ethereal and less grounded.
To compound this problem, electrostats have more diffuse imaging than dynamics, so instead of hearing sounds precisely pinpointed in space, they will seem to come out of a general area in space but you won't always be sure where they're coming from. This usually widens the perception of soundstage, but adds to the airy/diffuse feeling and the perceptible lack of solidity.
Which is why the SR-001 sounds so ethereal and diffuse at times, compared to a punchy, impactful dynamic. This is also why, when the music turns very difficult, said dynamic throws its cards in and folds, whereas the SR-001 gets its groove on. You really can't torture this headphone enough with brutally complex music, and same goes for every 'stat that I've heard thus far (SR-303, SR-404, HE90).
Balanced armatures have a lot of the speed of an electrostat, but they also have some of the impact of a dynamic, which is why I like them so much - when they're well implemented. To this day, though, the ES2 is the only headphone, out of what I've heard, which I can wholeheartedly say implements balanced armatures well. The weakness of this driver technology is high frequency reproduction - I don't think they've cracked 20khz yet; most crap out at 16khz in theory but well before that in practice, so you're missing some of the overtones. That gives a lot of instruments a plasticky, textureless quality and hurts soundstage. Interestingly enough, balanced armatures have the crispest, most precise imaging out of any driver technology I've ever heard (the ER-4S is especially good at this).
And how does the SR-001 compare to the HD650? Favorably when the HD650 is driven by mediocre gear, but when gear quality increases, the HD650 pulls ahead. The SR-001 simply cannot overcome the limitations of its rolled-off treble and diffuse sound. The SR-001 system is also burdened with an amp that, while portable, is really quite far from the last word in sound quality (though under AC power it's not bad). Still, out of the 007t, the SR-003 sounds like what the HD650 should sound like but rarely does - very fast, very warm, with a lush, euphonic but highly detailed character. I prefer it over the HD650 unless you're driving the latter phone balanced, or out of some seriously good single-ended amplification.
So, given how cheap the SR-001 is, I think that's a very high recommendation.
It should be noted that I prefer the HD600 over the HD650 though, and over the SR-001 too, simply because the HD600 is one of the truly few headphones that have managed to get tonal balance, instrument tone and tembre completely right. In fact, out of the phones that I've heard, only the K340 is anywhere as close on this, and still not as good. The ES2 is not bad either, but being a balanced armature it's missing some overtones, so texture and tone are hurt.
Hope this helps...
