Wow, fantastic information, thanks everyone. I had a suspicion that I was probably oversimplifying, so I'm glad I asked. There's good info in every answer, thanks to all.
Newbee, your point about buying only the power needed is very pertinent. I'm driving a set of B&W PM1, with 84dB sensitivity and they dip to 5.1 ohms. I went to the dealer to look at a Rogue Sphinx, certain that I needed every bit of their 100wpc (if not more) to push the B&W (my Marantz integrated with 70wpc did not match well at all and ran out of gas). The dealer showed me the Sphinx, then the Cronus Magnum with 90wpc, but insisted I also take a listen to the pre-owned VSi55 he had just taken in, with brand new tubes. I almost made the mistake of dismissing the ARC thinking that 50wpc would be a backwards step and waste of time and money, but I'm glad I listened to him. I'm too inexperienced to understand the technicals of why 50 watts of tube power opens up the B&W and pushes them to insane volume levels while the music remains perfectly clear and listenable, while 70wpc of solid state power turns to aural chaos at anything above 1 o'clock on the dial - I'm sure the Marantz is a superb integrated, and it was just a bad match-up. But the lesson I learned is that there's more to the story than just absolute WPC.
I like the thermometer gun idea, Viridian.
Jjrenman, your point about Class-D is duly noted - when summer heat rolls around, having one for back-up duty may save me electric bill $$ and make my den bearable if the ARC just gets too hot.
Al, thanks for the layman-friendly explanation - I needed it!
Eric
Newbee, your point about buying only the power needed is very pertinent. I'm driving a set of B&W PM1, with 84dB sensitivity and they dip to 5.1 ohms. I went to the dealer to look at a Rogue Sphinx, certain that I needed every bit of their 100wpc (if not more) to push the B&W (my Marantz integrated with 70wpc did not match well at all and ran out of gas). The dealer showed me the Sphinx, then the Cronus Magnum with 90wpc, but insisted I also take a listen to the pre-owned VSi55 he had just taken in, with brand new tubes. I almost made the mistake of dismissing the ARC thinking that 50wpc would be a backwards step and waste of time and money, but I'm glad I listened to him. I'm too inexperienced to understand the technicals of why 50 watts of tube power opens up the B&W and pushes them to insane volume levels while the music remains perfectly clear and listenable, while 70wpc of solid state power turns to aural chaos at anything above 1 o'clock on the dial - I'm sure the Marantz is a superb integrated, and it was just a bad match-up. But the lesson I learned is that there's more to the story than just absolute WPC.
I like the thermometer gun idea, Viridian.
Jjrenman, your point about Class-D is duly noted - when summer heat rolls around, having one for back-up duty may save me electric bill $$ and make my den bearable if the ARC just gets too hot.
Al, thanks for the layman-friendly explanation - I needed it!
Eric