I believe John Potis did a short comparison of Zu speakers and the Callistos in his review of the Zu Tones; I would imagine that the comparison would be pretty applicable to the Druids vs Callistos, except that the Druid would offer more resolution and more bass than the Tones, which I currently own.
Also, you could always send John an email; I've asked him questions before through email and he has always responded fairly quickly.
