Do you look at your music or listen to it?
I remember selling a mix of Harman Kardon, JVC and Fisher as a low end in the mid-80's. I used to show people how inaccurate power ratings were and how useless features were by turning up the JVC 500WPC amp (I know 500WPC! Sure, LOL) and then letting them watch as the lights flickered on the display as it pushed, creaked and groaned to deliver some heavily distorted output while making the lights dance on the fancy display window.
The Fisher, of course, was FAR worse still, but that goes without saying.
But the HK didn't HAVE any lights to flicker, except the little red LED for power on. And yet, while rated at only 100WPC (their top of the line then), it would run circles around the others and priced almost the same as JVC (driven with up to 60AMPS of current, that power moved FAST). And boy was the sound quality different than a fully featured top of he line JVC with all the 'tuning' bells and whistles. Features are not sound. Static wattage is useless for anything but lighting up LED's. Adding some current and actually MOVING that wattage is what counts.
So my very first question to people looking for a receiver was/is "Do you look at your music or listen to it?" Got a few odd looks for that one but told me what sound room to take them to.
If they are impressed with features, then obviously sound quality was not key and off to the JVC or Fisher rooms. If they liked sound, it was off to the rather basic looking, champagne finish of an H/K PM-665. no EQ, no dancing lights, bells and whistles, but some excellent filtering switches for crossover levels, discrete circuitry, phono cartridge type selection etc. instead. USEFUL features not just ones to make up for poor output.
Long and short, most mid-fi (pretty) receivers WON'T put out what they are rated at, unless cranked where they just may offer a tone or two at the rated power output, heavily distorted of course and ready to blow ANY speaker.
A good amp, can drive a speaker with a far lower power rating than the amp's rated output, as it delivers clean power without clipping and distorting. Or as others have said, too little power causes more trouble than too much.
So the amount of output power is actually pretty irrelevant, much more important is how CLEAN that power is. That's why 20W of Cambridge costs more than top of the line H/K receivers these days. Why a vintage 20W tube amp from MacIntosh sells for upward of $5k used. And why those cheesy retail products, with every bell and whistle added to create interest in features instead of sound, are so cheap and only available in big box retailers that sell to every unsuspecting customer to enter the doors.
A GOOD 20WPC amp will usually drive a 150W speaker at full power far better than a cheap 150W+ Amp (and sound better, louder and fuller). A cheap 150WPC amp will kill a decent set of speakers as soon as you turn it up, no matter their power rating.
WPC ratings on retail (especially big box)consumer electronics are absolutely useless to anyone but the marketing company and bean counters.
Of course these are my own views from personal experience, some people actually like all the lights and features added to boost price and elevate interest in receivers, whether they actually improve sound or screw it up.
I get dealers in here all the time asking for power ratings on RBH speakers that they then try to match with their client's amps. They just don't get it. It's not their fault, it's the industry's unregulated marketing efforts that MAKE people think these numbers have meaning, beyond simply increasing sales.
