Ok, I got my S2 today! First impressions with no break-in. they're really good. Quick notes:
The bass, similar to the EX600, has some good punch and extension. It's more impactful than the BA IEMs I've had before, which is something I wanted from this (and the EX600). And it's not muddy or boomy - it's nicely controlled and textured.
Mids are clear and detailed but aren't quite as in-your-face as the EX600, which presents them closer to the listener. The S2 seems to place me further back from the stage. For some songs, the EX600 mids are a little like Grados in that they really put you in the middle of the song and can be enchanting. The S2 mids don't do this. They don't jump forward in the same way. so it makes for a slightly deeper soundstage at the cost of some "excitement" and "richness" for some songs.
Highs are not prone to sibilance. it'll present it if it's there but it doesn't accentuate it for the sake of added sparkle on darker recordings. I find them detailed and well executed. Nothing stands out as shrill or tizzy sounding.
Soundstage is pretty good but it's not something that makes me think I'm wearing a full size open headphone. imaging seems precise and instruments don't feel smudged or streaked across in space. they have specific locations in space.
I'll be running these in over the weekend - not sure if burn-in will change anything. Figure it won't hurt.
One thing that's really important with these is the fit in the ear canal. I have real tiny ear canals and I always reach for the XS tips whenever possible. With other IEMs (including the EX600) moving up or down a tip size doesn't radically change the sound. but it did here. With the XS tips, I thought the mids were hollow and thin. I was worried because even though it felt like I was getting a good seal, I wasn't. I moved up to the small tips and the mids became much more full and natural.
The way the entire IEM fits in the ear, I could see the S1/S2 being uncomfortable if you have a small inner ear shape as it needs to make room for the housing and the large driver and still get the nozzle directed in the canal.
Anyway, first impressions are very good. For $150 these seem like an excellent choice if you can get the proper seal (a tiny, almost unnoticeable leak changes the sound more than other IEMs I've used).