The S0 seems to feature with a mild v-shaped sound-signature, with a very nice bass presence (mostly mid-bass, but a surprising sub-bass as well), balanced mid-range, and a slightly extended treble for added clarity. Overall, I enjoy this sound more than S1, because it's a lot less brash and agressive.
Your choice of tips will also impact your sound impressions. Without a proper seal you get some bass leak, but also a more balanced and airy sound. If the seal is good, then S0 becomes slightly dark, and warm at the same, while still retaining its clarity.
Given the right track, the bass becomes surprisingly potent. However, despite the enhanced bass presence, the songs still retain clarity, which is always satisfying to hear. Listening to something a bit more electronic, the sub-bass presence is noticeable, in many ways similar to B&W's C5. Other than that, you'll find that the emphasised lows are still tight and deliver a proper kick, the kind that makes drum solos entertaining.
Sadly, all that bass is not all good. There's a clear lack of texture to be had, and the mid-bass ends up sounding boomy at times.
When it comes to mids, these don't see the same level of attention as the bass. They're nicely balanced with a proper flow from lower to upper-mids. Overall, the midrange is warm, and clear, even though it remains slightly distant or recessed.
Treble is somewhat mellow, though it's still good enough to add something to the perceived clarity. I am not a fan of hot-headed, sparkly, airy treble, but even for me, the S0 feels at times a bit too forgiving. I guess that means even the brightest songs don't end up being offensive to the ears.
Used with large open-back over-ear headphones like the Sennheiser HD650, I found IEM soundstage almost tiny. Still, compared to the Shure's and B&W, BrainWavz isn't lagging far behind. The sound is still pretty much in your head only, but positioning is decent enough. Imaging also doesn't sound as good as B&W and Shure, but considering its price, I can't say it's flat out bad.
So yah, the sense of distance and space is not really there, or better yet said the stage doesn't feel that wide and deep, while separation / imaging seem to be good enough.