Question on speaker sensitivity / power.

In general, yes you are correct. Rather than get into all the science behind this, you will also need to factor in how big is the room that you will be setting up your system in, how loud do you like to listen to your music, and what type of music do you like to listen to. Also, you want an amp with more reserve power than a speaker's max. If you drive your amp to near clipping/ clipping, you will fry your speakers as the power is "dirty" power.

I have Opera Plateas that I use with a Prima Luna Prologue 5 (tube based, 36 wpc). Even though the room is pretty big by NYC apartment standards (15 X 23 X 9), I listen at low levels, so the amp choice works out ok. If I were listening to orchestral music at loud levels and wanted to hear all the details, I would most likely need a more powerful amp.

FWIW, the Opera Callas are rated from 50 to 100 watts. So, you have many choices available to you in an amp.

Regards,

Rich

very general rule of thumb...under 90db go ss and over go tubes but you must also consider speakers impedance....under 8 ohms go ss and 8 ohms nominal or higher is tube land....don't get hung up on mfg. power rating for speakers....it's almost meaningless
Larry

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.