I'm biased since I've been using the Callistos for the past ten years. All I can say are they are very involving and one of the most realistic sounding speakers I've heard especially on voices. I have heard the Thiels many times and I prefer the Callistos, just my opinion. Compared to the Callistos, the Thiels did sound bright on certain voices and recordings and it could be the metal tweeter, not sure.
I'm also using a Rel Storm III sub to augment the lows and this sub blends extremely well with the speakers. In my opinion you do need a sub with the Callistos to reproduce the full range, the Thiels don't.
The Callistos are extremely easy to drive and much more sensitive than other time/phase coherent speakers. I'm driving them with a Pathos TT 30 wpc integrated and they get much louder than I can ever listen to. I couldn't use this integrated with many other speakers.
My only debate now is whether I want to upgrade to the EOS HX.
Has anyone compared the Callisto with the EOS HX? I assume it will be an upgrade but probably more of an incremental upgrade.