Shocked. Need Opinions. How muck power do I need?

Great points made by everyone: Current, slew rates, transient power reserves, damping rates, bass control, not sweating low impedances, and being "musical".  But there is something about a large amp just idling along happy at putting out 40W to 100W peaks compared to a smaller 40W amp that will run out of steam - if you want to go louder. And the synergy with the speakers.  As Andrew Robinson stated in his review of the new Emotiva Airmotiv XT2. That thing was a power hog. It sounded good, but he had to step it up with what amp he used to get it to play nicely. I imagine Magnepans would be the same. 

Just sitting here watching the Mac 1.2 kw meters swing 1.2 watts 70 dB on the speakers so don't need alot of watts most times. It is the amps that drive the base tweeters don't need much on the voice coil and magnets smaller compared to offer magnets and voice coil then depends on crossovers electronic dsp or coil caps then headroom if you push an amp on upper limits causes risk of clipping or distortion damaging speakers so it is nice to put the ear muffs on and rattle the walls at 110 dB and feel the music pound you chest wall.some day try the crown I tec 12000 watts on pro jbl.enjoy the music power is volts x amps

It depends. All amps and watts are not created equal, all speakers and speaker impedances are different, and our volume preferences are different. You only need enough power to drive your speakers to the volume levels you prefer in your room. Headroom is nice if you push dynamic passages to higher volumes, but most of the time I don’t come close to the limits of my amps.

Even with speakers of average sensitivity (~ 89db), over the years I’ve gone from a high of 255 watts per channel (Hafler DH500) down to about 17 watts per channel (tube triode), and my current amps sound much nicer to me than any other more powerful amps I’ve owned. In fairness, for bass heavy passages an ample size SS amp tends to handle big bass peaks on large woofers better than my small tube amps do, but that’s one aspect of the overall sound spectrum (below 80hz in my case)

one of the rare threads in which everyone seems to be making good points. my own experience, which matches other posters', is that high current delivery is by far the most critical factor--even with demanding speakers like dynaudio i've heard 50w pass amps outperform amps with much more wattage.

@loomisjohnson  - I can believe that, but also Pass I think likes big old school speakers.  See if you can find Pass' article on damping factor and whether it really should be low or not.