TL;dr: Color me impressed.

I got to hear these as part of the IE600 tour, and they're now on the way to the next listener. Thanks @ericpalonen for the loan. Opinions here are subjective, unvarnished and unquantified.

I'm not going to bother with the company, packaging, accessories. I'm pretty sure you've heard of Sennheiser, and there are plenty of photos around. You're certainly not getting these IEMs for the box.

Specs from the Sennheiser site:
Transducer: Single Dynamic Driver, Pressure Chamber
Frequency response: 4-46,500 Hz
Sound pressure level (SPL): 118 dB at 1 kHz, 1 Vrms
Total harmonic distortion (THD): <0.06% (1 kHz, 94 dB)
Cable length: 125 cm
Connector: Fidelity (+) MMCX
Impedance: 18 ohm system impedance
Attenuation: 25dB

You'll have to ask somebody's dog about that 46.5 kHz frequency response-- I'm only human!

Let's talk about what matters most: Fit, Cable and Sound.


Fit

These are the same form factor as my own IE300 (and other Sennheiser IEs). It's a small oblong/rectangular hybrid that should tuck into just about anyone's ears easily. With metal housings, the IE600 are a little more substantial in the hand than the plastic IE300, but they still pretty much vanish in the ear. Light and unobtrusive.

But there is something unusual about the way Sennheiser IEMs fit.

With most IEMs, I use large eartips and push them in as deeply as possible to get a good seal. Sometimes foam, sometimes silicone, but without a deep fit there's no isolation or bass for me.

These are different. I tried a lot of my other tips with these, but I came back to Sennheiser's included L silicones because they are the only ones that worked for me. (The package includes S-M-L silicones and foams).

There's a trick with the IE300 and with these. The Sennheiser tips have an extra groove inside their sleeves, so you can put them on the nozzles in two spots: pushed all the way onto the nozzle or about 2mm up from there.

Large silicones, on the notch away from the base, were the only ones that worked for me -- and absolutely everything else sounded tinny and pathetic. You get a seal or you don't. Also, and crucially, instead of deep insertion with the silicones, I pulled back slightly once they were in my ears. There's exactly one spot where the soft silicone completely conforms to the ear canal. And at that spot....yes!

Just don't expect to seat these like your other IEMs. They work their own way to get the ideal fit.

Also, you need to rotate them just so on the MMCX connection. The cable has a stiff memory hook around the ear, and it can fight with you, trying to pull them out. That cable!

Cable

Sigh.

The 3.5mm SE "para-aramid reinforced" cable looks and feels the same as the much-derided IE300 cable, and the IE600 also comes with a 4.4 balanced cable of similar construction. (The para-aramid family of plastics includes Kevlar, but perhaps for trademark reasons Sennheiser doesn't use the word.)

I hate the cable. On the plus side, it's light, and Sennheiser has said it's made to be durable; the same cable on my IE300 is going strong after more than a year. BUT: It is seriously microphonic and it has nonstandard, recessed MMCX connectors. So you're stuck with the noisy OEM cable unless you want to pay $$$ for one of the few aftermarket replacements -- and then hope that they are less microphonic. Why Sennheiser made this new unit with the same annoying cable baffles me.

Using the 3.5mm cable, I listened to Tidal Masters and my own downloaded or ripped 192k/wav/FLAC/mp3 files on Foobar2000 and Vox, both from my MacBook Air headphone jack and via my VE Megatron DAC/Amp. I also played FLAC, hi-res wav, CD-quality wav and mp3 via 3.5mm SE from my A&K AK70.

With the 4.4mm cable I listened to Tidal Masters and my offline files via the VE Megatron. There's plenty of volume without amping. The sound from the Megatron was an improvement -- more open and realistic -- but I can't specify whether that's a result of the DAC or the amping. Single DD with 18 ohm impedance probably didn't need an amp.

Considering the cable changes and volume matching it would require, I'm not set up for any detailed comparison of SE and balanced. Perhaps someone else on the tour can do that.


SOUND:

Ahhhh, yes. It's impressive. Without looking at graphs, I had guessed that it's a W tuning: deep bass, upfront vocals, extended treble.

I threw a lot of things at the IE600: African and trip-hop subwoofer madness, Bach and Stravinsky and Messiaen, jazz, organic roots-rock and Americana, thrashing metal, vintage soul, future-R&B, sparkling Laurel Canyon productions, low-fi indie-rock, high-density prog-rock, New Orleans brass bands, subliminal-noise Nine Inch Nails, extreme-bass Hans Zimmer soundtracks, grimy low-fi hip-hop, otherworldly ambient, whisper-to-crash Billie Eilish.

I also ran the test tones from audiocheck.net -- no big peaks or troughs --- and, for spatial cues, tried the Abyss video on YouTube.

It's hard to trip up these IEMs. Not impossible, but not easy.

Bass is extended and precise; the lowest notes retain pitch as well as impact. The bass drum that opens Feist's "The Bad in Each Other" goes straight to the solar plexus, and so do the thuds and swoops of Amazondotcom's "Gut Ritual." In "Angel" by Massive Attack, the pitch is undistorted enough to clarify that the bass notes at 0:07 are microtonally lower than the ones that start the song.

Vocals, male and especially female, seem slightly pushed upfront, along with midrange instruments. If you're trying to pick out lyrics that's often a plus. David Bowie's "Sue (Or in a Season of Crime)" surrounds him, in the same register as his vocals, with a jabbing guitar riff and a saxophone panning around the mix. But his voice still holds the center. In a weird mix like "Here She Comes" by the Beach Boys (from "Carl and the Passions -- 'So Tough'"), the lead vocals in the later verses (around 1:54) aren't exactly prominent, but the IE600 gives them a fighting chance.

Beth Orton's voice hovers unforced above the piano and percussion of "Haunted Satellite." Billie Eilish's "Bad Guy" really skulks along in the bass, yet without any intrusion on her breathy voice ("duh!"), and the IE600 also beautifully details how staggered all the overdubbed fingersnaps are. It's even possible to decipher Thom Yorke's rant in the Smile's "You Will Never Work in Television Again."

A well-recorded string quartet -- how about the Emerson playing the Allegro attacca from Bartok's String Quartet No. 3 -- sounds bracing on the IE600, and brasses and saxes gleam.

The IE600 puts some air around each instrument; there's treble extension to match the bass impact. Transparency is the priority, more than warmth/blend. Personally, I'm a big transparency fan. A piano-and-percussion duet like "Fititi Nongo," by the Cuban pianist David Virelles, neatly delivers all the rhythmic crossfire it should. Everything Lindsey Buckingham picks and strums on Fleetwood Mac's "Go Your Own Way" shows off distinct timbre and transients, while the backing vocals hop in and out.

In Paul Simon's "Can't Run But," the fitful percussion and J. J. Cale's guitar curlicues glimmer throughout. The percussion that's spread across "On the Corner" by Miles Davis -- trap drums, tabla, cowbell, congas -- is differentiated and distinct. And every scurrying piccolo and clarinet in Valery Gergiev conducting Stravinsky's "Rite of Spring" streaks brightly through the pagan wilderness.

But there are also downsides. For styles like thrash and death metal that feature high-speed, distorted rhythm guitar, the IE600 seems to spotlight the buzzing top end of the distortion instead of the thrust of the chords. Or in a song like Sylvan Esso's "Moving," with a lot of sizzling synth tones, the IE600 seems to emphasize the hissiest frequences. The IE600 isn't going to smooth out any extremes of a recording. And if you're sensitive to sibilance, the IE600 are likely to trigger you unless you dial in a different EQ. (I didn't play around with EQ, though I would if I owned the IE600, taming the treble and maybe rolling off a tiny bit of bass.)

The bright treble improves positioning, of course. Some three-dimensionality was audible when watching the Abyss video, though more width than depth and more depth than height. (Too bad the lossless download from the @Abyss YouTube page doesn't sync with the video.)

Listening to music files that are better quality than YouTube, there are plenty of spatial cues -- certainly in the round if not fully spherical. "Lonely," by Koffee, has that thick reggae bass, but it never masks the rimshots or hi-hats; tom-toms roll across from left to right, and the placement of organ, piano and backup voices is well-separated and precise. In "Jele," from the "Black Panther: Wakanda Forever" soundtrack album, I turned to look around at 0:59 where Busiswa suddenly shouts her entrance.

All in all, the IE600 is an IEM that makes good on its obvious intentions: full-spectrum response with an emphasis on revealing as much as possible, plus a little bit of fun at the extremes.

Comparisons: Well...I don't have any other IEMs in this price range. My Venmo is...j/k.

Sennheiser IE300: Simple -- the IE600 outdoes the IE300 in every way (as it should at 3x the price). I like the timbre on the IE300, but it has a pushy, exaggerated bass. That's especially obvious when compared to the IE600, which hits the same notes but keeps them more in proportion. The IE300 doesn't match the lucid high end of the IE600, either; it sounds more constrained. I guess they had to save something for the upgrade. For what it's worth, the vent hole is differently shaped and differently placed on the IE300 and IE600, which may account for some of the increased openness.

Tri I3: Included just because they're my other favorites among my IEMs, the original Tri I3 (not Pro) make an interesting contrast to the IE600. They're a tribrid -- DD, BA, Planar -- that begs for more power than the single DD IE600. With a more restrained treble, they don't spread the instruments as widely as the IE600 or provide the last bit of crispness on percussion. On the other hand, despite the multiple drivers, they do seem to bind a mix together. I'd say the Tri I3 is the huddle, while the IE600 fans out the instruments for the play.

The list price of $699 is a serious investment, and without comparing others in that price tier I can't say whether the IE600 is a good value proposition -- though it's a good-sounding IEM. I'll also note that the Sennheiser IE300 quickly dropped in list price from $300 to $200. An equivalent price drop for the IE600 would take it down to $466, which wouldn't be a bad thing, so you might want to wait a bit before pulling the trigger. (It could help pay for replacing the cable.)

All in all, a very enjoyable sound, nicely positioned between analysis and oomph.