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

What amp weighs 300 lbs and makes 4 watts?  

Lots of really good content here. In my opinion, there are definitely several factors, besides the more obvious speaker load, room sizes, listening levels, etc.

Current can be very important and power supply design is a major factor here. I am not of the mind that more power is always better, particularly within a designer’s range of products.

i have a Pass XA25 driving a pair of Dynaudio speakers in a decent size room (though I do only sit about 8ft from the speakers or so) and for me, the Pass may have run low on steam once. 

Many, many moons ago when I worked in a high end store, it was not uncommon for one company’s lower wattage products to have some qualities that bested their larger wattage amps. Sometimes this was timbre in particular, but sometimes soundstage as well. Bass control on larger speakers does usually require more wattage. 
 

Now with very sensitive speakers and very low wattage tube amps, that is a different thing of course…

That is a huge space for those speakers, and the impression of diminished bass is relative to prior powerful amps, perhaps making more bass out of those pairs of 6.5", but with more distortion than you are/were aware of.

I would want larger woofers, (I have and would want 15" in that space). I always like mid and tweet horns, then, how much power to drive them? And, you say solid state, but I would want tubes, made affordable and more practical by efficient horn speakers.

My 14 x 24 room, I use 45 wpc tubes, that never seems 'not enough', but I don't listen to cannons and crashing highs anymore.

Definitely consider more power than generally needed to have reserves for instantaneous demands.

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