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.