Some brief impressions of the A18s relative to the A18t...
The 18s is definitely built off of the T's DNA. It still possesses all the fantastic technical attributes that the t has with its superb imaging, separation, and sound stage.
Where the 18s differs to my ears is that I hear more impact on the lower end and better quality relative to the T. I also hear the mid range to be a bit more forward and lush. Vocals also don't sound as dry on the 18s. The sound is slightly thicker overall relative to the T.
Overall they are quite similar in many ways but I think the S with its slightly different tuning approach will be a more favorable option for many. It hits a sweet spot between musicality and still being a reference monitor. Very impressed so far... 2 thumbs up from me.