Soundwise, I'd characterize these as "basshead," in the sense that their defining characteristic is their voluminous, deep and impactful low end. Subbass and midbass are well-controlled and quite fast, if not as well-articulated as the best sets, and there's little bleed-over into other frequencies. However, the sheer quantity of the low end gives the ZS3 a slightly less-than-seamless quality, as if you're listening to a separate subwoofer rather than perfectly integrated drivers. In this regard they remind me of the Velodyne V-Pulse, another good IEM titled a bit too much to the bottom end. I should probably try eq-ing or trying different tips with the view to better balancing the bass.
Mids are very rich and clean-sounding; voices and guitars are presented with a lot of body but still sound quite natural. Treble is a little warmed over but likewise has a lot of body and good clarity; it's not especially extended or detailed, but neither is it sharp or strident. Resolution is not close to the level of say, the XE800, tho drums, piano and other quick transients are quite well-rendered. In general, the ZS3 seems to be going for a big, expansive sound which eschews microdetail and intimacy for more excitement.
Where these really excel is in soundstage and imaging/instrument placement, which (like the ED9) is incredible--you can place the location of each musician precisely on a very wide stage. In this regard they remind me of the Tennmak Pro and Piano, two similarly-configured IEMs which also present a very big, accurate stage. However, the ZS3 has better-controlled bass than the Pro or the Piano, and as a result perhaps better overall clarity (the Pro and especially the Piano do have more highend resolution/detail, however).
Compared to my favorite KZ, the ED9, the ZS3 are warmer and has more and tighter bass; the ED9 sounds considerably smaller but more transparent/natural.
