GMA Callisto's

do the Callisto's require much power? in a medium sized room at average volume, could one get by wih say between 15 and 30 watts??
I would say yes. They are around 90-91 dB efficient, with a very flat impedance. I was able to run a pair with 3.5 watts, and they sounded incredible. They just wouldn't play at loud volume, but I could have been happy with them. They were on loan from a friend, so I could hear them in my room with my amps.
I own the Callisto's and first drove them with an AES Superamp with 15 watts with great results. I have moved up to a Cary Rocket 88R with 30 watts and they are fabulous. Both drove the Callisto's well, but the Rocket 88R is 10 times the amp IMO. The Callisto's are still the most enjoyable speakers I have owned.
I am using a Kr Antares VA300 with the Callistos which puts out about 12 watts. They play loud enough for me. I live in a 1 bedroom condo with a 20x25 living room area and can't play them at high levels without bothering my neighbors. When I first received my Callistos I played them loud to break them in, you could hear the music outside my unit in the hall and a neighbor left me a note asking that I turn the volume down.