I am currently using a pair of Theta Citadel 1.5, and an ARC Ref 3. ; These speakers do not need a powerfull amp, but you need an amp that can go quite low. Focal demos them with Halcro but I did not heard the combination.
I heard them with Krell but did not find it was a good.
I heard them with Viola amps : very liquid but did not go low enough for my taste (some meat was missing).
This speaker is very fast and can go very low. So you need an amp accordingly. After 5 months of experiments, I can say that these speakers are so revealing that you need very carefull matching, including all types of cables.
I do not feel the need to change my amp, but as you know it's a never ending story... I am aware that I have room for significant improvements but limitation is definitely not the speaker in my set up. At this point, I still have an image too big for voices and violin (for instance), but experiments with new cables showed that it could be solved.