I'm considering the same combo, so an curious as well whether anyone has heard this. I've read the review in Hifi world, and also the glowing 5-star review of the S-1 in Hifi Choice, which you may have read as well (dec 2015 and 2015 yearbook). Hifi Choice says that the usual 'politeness' of the Quads has given way to more expressiveness and immediacy. The S-series seems to be received quite well. I also noticed a positive review of the S-5 in a German magazine.
For what it's worth: I already have the Quad Vena, presently combined intermittently with Dali Zensor Pico's and Cambridge Audio Minx 21 + sub. Of course the Vena can drive larger and better speakers, which is why I'm looking at the S-2 (besides possibly Harbeths). At the dealer I heard the Vena set up with Quad Z-2's and with Guru Juniors. The Guru Juniors sounded nice, to me they resembled the Dali Zensor series. However, the Z-2 was clearly in a different class, being larger and having more bass, but also for being more transparent, clear, refined. As far as I understand, the Z-2 uses a similar or the same ribbon tweeter but is a bass reflex speaker. It is also larger and twice as expensive as the Quad S-2.
With respect to your interest in the Marantz/Dali combo: I've never heard a Marantz so can't help you there. I personally prefer the Vena solely on looks. The Dali's appear to be excellent speakers according to the numerous reviews. The Zensor Pico I find already fine-sounding with the Vena, even in the bass region, while they are really tiny and low price, so I imagine the larger Zensors should be very satisfying.
My interest in the Quad S-2 is driven partly by looks, partly by precisely their reputation for 'politeness'. My impression (also from hearing the Z-2) is that the Quad speakers try to provide a very satisfying listening experience for classic, vocal, and 'softer' rock music, particularly if you don't care for playing at extremely loud levels. Which is exactly what is my style of listening. This is not to say that the combo wouldn't work for hard, thumping bass. I just didn't listen to that kind of music on the Vena/Z-2. But I could imagine that there are other combinations that would be more suitable to such styles.
Before the Vena I had a cheap Denon Class D amp, which, although initially sounding impressively detailed, quickly gave me listening fatigue. The Quad Vena is much more comfortable while to my ears not less detailed.
Hope these personal impressions may help you. I agree, though, that you really should audition them. I probably won't be able to do so for at least a few months.