How many watts do you really need?

Power is voltage x amps in physics. Big capicitors store energy like in the furman 20 i has 60 amps of current reserve for transients in large monoblock amps. There is some justification in 220 volt amps as watts equal volts x amps. Some krell amps have that option. The power supply in your amp matters as big power supply can produce many amps. Some of my krell monoblocks have 2 huge transformers thus > 100 lbs each.the mcintosh uses big transformers they call it them autoformers to  produce large amps. Mcintosh 1201 1.2 1.25 all produce 200 amps and drive the large speakers with big voice coils and big magnets ,multiple drivers. I would never put them on my voice of theater because they don't need that power same with my klipsch.this is where 10 watts works well. As alot have mentioned headroom and transients. This limits clipping that kills speakers inotherwords distortion.  The more headroom you have for the kick drum the better. My ribbon,planar speakers can drop below 1 ohm and an amp has to be stable in that arena.  So you but the speaker then find a synergistic amp with matching gain for pre amp.but if you change speakers in the future upgrade you can buy an amp with much more wattage to begin with. Now what about class d as my d sonic has 1.5 in 8 ohm 2.2 k in 4 ohm pascal module. Runs cool.then you have ganfet class d. So many choices today. Do your homework find what you like and enjoy it.😃

My amps are class A for the first 10 watts and I rarely use all 10, but the heat they put out is enough to keep my AC running. 

Worth considering the full story with more power is that smaller drivers must work harder for bass but the  smaller voice coils also tend to heat up more with more power which increases resistance and can affect dynamics. So larger more efficient speakers have an advantage on this front. 

@elliottbnewcombjr 

15" woofers, weighing 37lbs each

I can’t say I have used or seen a vintage with large woofers such as 15" that could go below 30Hz, doesn’t mean it doesn’t exist. It was more about SPL in those days than frequency range. Vintage Klipsch Cornwall with 15" woofers = 34Hz.

Yes, small stand mounts are more demanding, but it’s not because they have more frequency, it is because they are less efficient/lower sensitivity.

Demanding is a bit off of a term because towers are more demanding. Sensitivity is only from 300Hz - 3kHz. The B&W 805 D2 bookshelf has a 88dB sensitivity but my 802 is 90dB. The crossover is 350Hz and the biggest difference is the woofer and cabinet size. So 802 gets the benefit of 50Hz (350-300Hz) added to the sensitivity rating. Also sub bass on the 802 can be around 15Hz. You need more power to move heavier drivers but not as much as one would think because they are not high excursion drivers like a subwoofer. That’s why I see towers more demanding. btw, the 802 is A LOT louder than 2dB over the 805 if sensitivity was really a full range measurement.

Overall, I would have thought it’s as easy as having a meter on an amp to tell you how much power is being used, but apparently it isn’t. I have yet to hear from a professional in the industry that can definitively answer the question. On another note, I’d be curious to try the meter on the Laiv GaN amp on my B&W 802 and see what it shows.

@bartsw 

"Don't vintage speakers have poor frequency range? I can't imagine them to be as demanding as newer speakers with growing interest in bass."

I gave you a 'general' answer, I didn't realize you were specifically focusing on very low and very accurate low bass.

Below 30/40 hz at spl equal to the rest of the frequencies, without distortion: old or new you are talking about a swarm of subs, and they will have their own amps, and a large space, that's not what the discussion is about.

It's about amps generally being more powerful than we need for primary L/R speakers?