SCORE: 4.75/5 stars. Quite excellent. ALL negative comments are me being very picky.
It's not a cheap IEM, but it's also not in the most expensive price bracket either. So I think it punches WELL above its price.
Build: Insane. Gold-plated brass shell with real mother-of-pearl? It's a VERY nice design and very beautiful too. They aren't too heavy in my ears, although I'm using Spinfit CP100 eartips. It's a fairly comfortable fit, although not the MOST comfy IEM I've tried. The cable is cloth-covered so there's a lot of noise when it moves. I am using a different cable.
SOUND: a kaleidoscope of color. That BC vent really, REALLY adds super stage effects, so much more than the only other BC set I've tried, the Fan 3. Stage is surreal. Vocals are placed about inside your head, toward the front.
That being said, the tuning might be divisive, and I wouldn't call this an all-rounder, which is a tad annoying for the price.
Bass: CLEAR, rich, and thumpy, but not bass-head quantity. Sub-bass is rolled off a little, so it's not a prominent as I personally would like, but it can still REALLY dive deep when called upon.
Mids/vocals: very clear and holographic, but the overall treble presentation is on the bright side, so on songs that favor a warm playback, the Nautilus can make it sound a tad thin. That being said, there's absolutely no muddiness or congestion on busy tracks, which is a huge plus for me. It's not a "warm" sound, for sure. More crisp and accurate.
Treble: a treble-head's dream. Airy, DETAILED, with great timbre. There is some slight sibilance on songs prone to it, however. But if a bright, exciting treble is what you're after, sacrifices must be made.
Overall sound: The level of capability for jaw-dropping detail and texture while maintaining a reasonably great timbre means this really does sound heads-and shoulders above mid-fi IEMs, even heavy hitters like the ISN H60. I find I prefer it for instrumental music, however. That BC driver really works its magic here. Instruments sound so REAL. I mean, the vocals are great too, especially female vocals, but occasionally that can sound a tiny bit thinner than I think they should, especially male vocals. But it's nothing a warmer cable/source couldn't improve. Definitely pure copper or gold-plated copper, NOT silver-plated is my recommendation. And I'm being rather picky here, so take all that with a grain of salt. The fact stands that for someone who has used budget IEMs for years, and recently tried out some decent mid-fi IEMs, the Nautilus was a complete sonic upgrade in ways I never heard before, and I'm very impressed and happy with this purchase. I just know the tuning won't fit everyone.