Watts! How many do we need?

"I was using a 10 watt class A amp to drive a pair of Super Tablettes (86 db I think) in a 13x14 listening room with zero issues. So then I thought, "What the hell!" and put in my 2A3 integrated amp. Still no issues and the speakers sound great."

ProAcs are rather famously amenable to lower-powered tube amps. My first audio heartthrob was a pair of Studios with the little Cary stereo 300B amp I heard in a shop. I was thrilled a few years later when a friend sold me his Response 2’s. Thirty years on, with one set of replaced drivers and two sets in storage, I’m still enjoying them. ;-) I used 300B amps with them for years but the quest for better bandwidth and slightly more power led me to my current 20 wpc Williamson amps.

Current and output impedance are important as well. Paul from PS Audio gives a good explanation here.

@dogearedaudio 

My point was that I have seen systems so expensive, so complicated and striving for so much power and scale that they become unmanageable for the owner.  They're often difficult to listen to as well.

Point taken. But in this context we are talking about modern power amps/integrated amps that put out high-current, 300 wpc plus into a pair of small to medium size speakers. And in the case of Harbeth's, were designed/voiced for SS. 

Conversely, I'm hoping to put together a much lower powered tube system again when circumstances allow it, but maybe with some vintage Tannoy's or similar. (I owned a 35wpc Raven Audio Osprey that was oh so sweet sounding - I miss it)

No question both high-power and low-power strategies can sound fantastic when the synergy is right. 

Sanders has a vast knowledge I have some of his amps and speakers they are great.  As stated power is volts x amps.its the transients that take alot of watts.its the clipping that causes damage to speakers.it is well known esl take alot of power but big cone speakers do also. Some speakers are efficient that's why the reference 1 watt produces spl sound pressure level. As a standard to compare. There has to be some relationship to reference to be marketable.i have read some high powered amps don't play well at low volume but I have not experienced it. The class d amps are getting better and have alot of watts for lower price.they are put in many speakers these days.i like big speakers because I have the room.the huge watts bring them alive.i have alot of esl and planars from many decades.alot of speakers are putting ribbons in now.some of them have 32 ribbons each and require lots of power.paul mcgowen put a 1k watts each speaker into each of his irs v woffer tower they have 6 each colum.enjoy what you have and the music.stay healthy