Jim:
I've now got about 150 hours on the Callistos. Right off the bat, they far outperformed the Europas in clarity, detail and bass extension. Two days ago, they bloomed and the sound is unreal. They gained considerably in bass extension, space and air. Spooky! The bass is a bit flabby on the bottom-bottom, but I expect that will tighten up a bit with more time. They measure flat to 20 Hz with no TacT room correction, which they shouldn't. I'm rolling them off a bit below 40 Hz until they're fully broken in. I haven't used my VMPS sub since I put them in the system. In a few words, actually Roy's, "they kick the Europa's butt". I suspect, given their shape, they considerably outperform the Super Europas which use the same drivers.
Larry