Actually the max power rating(w/o clipping) for the Callas is 250W(RMS). That's IF this is the Callas you are referring to(there are others): (http://www.hifigear.co.uk/site/scripts/product_browse.php?product_id=4594) If you were shopping for SS amps to go with them, most would probably double their 8 ohm output into the Callas' 4 ohm nominal impedance. With tubed amps(if using output transformers) the output will be less into a 4 ohm load than into 8. Everything Rich says about room size/music genre applies. As long as you're not trying to listen to philharmonics at concert levels, 50 to 100 watts should suffice.
Question on speaker sensitivity / power.
I was looking at some Opera Callis bookshelf speakers and noticed that they are rated for 50 watts max and have a sensitivity of 86db. I would think speakers with that low of a sensitivity would actually perform best with a more powerful amp. Am I off base on the power/sensitivity issue?