I've pulled the trigger and ordered a pair from Amazon. I simply had to try them myself.
One things must be said loud and clear: Sennheiser's own silicone tips for the IE series suck. They're absolutely terrible. I've tried the IE80S with the standard pre-mounted medium silicone tips and it sounds nothing like it should. Boomy bass, heavily recessed mids, and shrill highs with tizzy cymbals and even sibilance in the vocals. Simply terrible, sounds like you'd expect a counterfeit to sound like. For some reason, Sennheiser dropped their hard foam tips and only provide single- and double-flange silicone tips - none of which ever gave me a good fit with the IE8 and IE80.
Luckily, they do provide Comply tips of all sizes, so in spite of my gripes with Complys, I resorted to using the small ones.
With the Complys, the IE80S finally does sound like the Sennheiser that it is.
Whatever you think about its sound signature, there's one thing I hope everyone can agree on: the soundstage is GIGANTIC. Like the IE8 and IE80 before, this is a soundstage so expansive that I've never heard anything remotely comparable from other IEMs. It's wide, very wide, and after a while it's like you're floating in it.
This, of course, may come at the cost of intimacy. If you like your sound to be very upfront, this is not the IEM for you. The IE80S is smooth; there's really no better word to describe it. The bass is, to me, fantastic. Yes, I can see how people used to neutral sound signatures can easily find it to be excessive. I myself think it's a bit exaggerated with some recordings. But it's warm, bouncy, engaging; it gives body to the sound, where I find so many other headphones are missing something. It's truly what the "base" of the overall sound signature should be.
The vocals are clean and precise, but yeah, definitely a little recessed. "Bathtub-shaped sound" is excessive criticism in my opinion, but vocals could easily be that little more upfront and nothing would be lost - quite the contrary, actually. They do get a bit lost in that expansive soundstage. Vocals are there, but not close to you.
As for the highs, they're... smooth. Present, fast, energetic, but very controlled. No hint of sibilance, cymbals have nice detail and snap but they'll never hurt your ears if you're sensitive to high frequencies.
Yet, the overall presentation of the IE80S is fun. I've tried other Sennheiser models such as the original Momentum, and even the HD650. The HD650 I found to be simply boring: smooth to the point of having no energy and snap at all, and with insufficient bass emphasis. The Momentum was great in the lower frequencies but it had virtually no highs at all: very warm, very smooth, but too slow, incapable of giving the slightest energy to any track.
The IE80S - like the other IE8x Sennheisers - to me is just right. It's really fun, always smooth, never shrill, and pretty quick.
I'm still on the fence about the IE80S being a worthy upgrade to the IE80. It's less flashy, less plasticky, and the earpieces are slightly smaller, providing better insertion and fit, but it's still anything but ergonomic. I don't know why Sennheiser went with this design when so many other IEMs come in a much more ergonomic shape.
The cable is thicker, but still not stiff enough, and it still doesn't have a memory section at the end. The IE80S also has a straight headphone jack, rather than the L-shaped jack of the IE8 and IE80. I don't know why straight headphone jacks are back, some years ago everyone on Head-Fi and audiophile sites was clamoring for L-shaped jacks. Maybe it's about smartphones dropping the headphone port and providing dongles, where L-shaped jacks would put more strain on the cable?
Anyway, the IE80S is quite expensive and the sound doesn't seem to be that much improved from the previous model. I still haven't made an A/B comparison with the IE80, but I may end up returning the new model as it does sound quite similar and I was more than satisfied with the original IE80. Both offer something that you can't find in any other IEM in the same price range, but I can see how they're not for everyone.