This and That:
I am glad that I've bought my SD-2 quite some time ago as nowadays, it doesn't come with the nicer looking silver cable anymore, and the design/materials and price have also been updated since then.
Back when I bought mine, all you got was the in-ear, a rugged storage/transportation case, a 3.5 to 6.35 mm adapter, two cleaning wipes and three pairs of silicone tips. It seems like nowadays the SD-2 comes with a proper packaging, but I don't really give a fuck.
The shell design is simply perfect and provides excellent fit and ergonomics. Seal and isolation are great as well.
As far as I'm aware, the BA woofer is back-vented.
Sound:
I'm only using my SD-2 with the largest included single-flange silicone tips as well as with low impedance sources. I don't intend to change anything about that.
Tonal Tuning:
The SD-2 is heading into a natural interpretation of neutral. It's basically got a bit more warmth and more darkness than what would be neutral/flat, hence it's a slightly mellow, natural interpretation of neutral.
Compared to my UERM/the UERR, the SD-2 has got ca. 2 to 3 dB more bass quantity and warmth, whereas compared to my Etymotic ER-4S/the ER4SR, it's got about 4 to 5 dB more warmth and bass quantity.
The bass extends flat into the sub-bass without any roll-off, but it also extends over the entire fundamentals and only stops around the lower mids wherefore voices gain slightly more body.
When it comes to the treble, it's evenly in the background although at the same time very even and doesn't show any peaks or sudden dips, except for one small lift before 10 kHz that however still remains below neutral in level.
Quite remarkable is that the super treble extension past 10 kHz is excellent and that there's basically no audible roll-off until 16 kHz. And that's achieved using a single-bore, dual-BA setup, whereas my Shure SE425 that's also a single-bore, dual-BA in-ear rolls of much earlier in the highs, which is also easily audible.
Resolution:
For a dual-BA in-ear with back-vented woofer, the resolution of the SD2 is very decent, but cannot quite fully keep up with most better triple- and quad-BA in-ears in the same price range.
The detail resolution on its own is very good and a bit above that of my Shure SE425, but performing critical listening, it can't fully keep up with other similarly-priced in-ears I in my possession, such as my Fischer Amps FA-3E, Pai Audio MR3, Audio Technica ATH-IM03 and others, once fast and complex music is being played.
Medium-priced in-ears with dynamic drivers are however usually clearly beaten by my SD-2 and the detail resolution is really by far not bad, but also not outstanding, but only "somewhat better than good" in view of the competition.
A major reason for that is the bass that is definitely on the softer and slower side for BA standards, and rather mellow sounding. It's still precise and the added body is nice at times, and it usually beats several dynamic driver-based in-ears, but for a BA lover like me, once faster and more complex music is being played, the control and speed as well as tightness are somewhat below what I'm used to and what I demand. Similarly-ish priced in-ears such as my SE425, FA-3E or MR3 are just tighter, quicker and more responsive, just better controlled in the lows playing fast and complex recordings.
Those who don't like the "typically clinical" bass that's often attributed to BA-only in-ears will however likey love the more dynamic character of the SD-2's woofer implementation.
As a result, I'm not using my SD-2 for very fast and demanding recordings but for normally fast and slower ones.
What the SD-2 does very well though are homogeneity and naturalness. These two attributes compensate for the only "good" resolution and rather soft bass response for BA standards. There are simply only few in-ears in this price range, and also in general, that are as natural, even and linear sounding as the StageDiver SD-2, especially in the treble. There's simply nothing about the tuning that is annoying or bad, which is thanks to the absence of undesirable sudden peaks and dips.
Another positive aspect of the SD-2 here is that it resolves equally well over the entire frequency spectrum, unlike my Logitech UE900 or Westone W4R, two in-ears where the midrange appears somewhat cloudy/less detailed compared to the highs and lows.
Soundstage:
Until that point, one might think the SD-2 is perhaps a little too expensive for the performance it offers despite its quality finish, but all the negative points are now blown away by the major core strength of the in-ear: besides the already very high naturalness and evenness when it comes to tuning, InEar's in-ear has got a very large, three-dimensional and quite precise soundstage that sounds highly authentic.
The stage is very spacious and at the same time very deep and generates a large and extremely three-dimensional space. The SD-2 layers very well and has the ability to work out individual layers in the depth plane very well and precisely. Instrument localisation as well as placement are precise and very authentic as well.
In this price range, the SD-2's soundstage is rather a unicorn for BA-only multi-driver in-ears, at least when it comes to three-dimensionality and realism. Those who want to find a multi-BA in-ear with an equally spacious and realistic, three-dimensional soundstage need to spend more than $1000, although then they'd find models with even more precise separation. But in its price range, for a multi-BA in-ear, the SD-2 is definitely quite unique and especially very convincing when it comes to soundstage.
Conclusion:
Natural and very even tuning, excellent extension past 10 kHz, very authentic, somewhat soft and slow, three-dimensional and large as well as authentic soundstage, great ergonomics.