Rated power (watt rating) relates to average sustained output level - the amount you can turn up the volume dial before distortion kicks in.
In general yes, but need to be more specific than just "rated power", such as rated power: 100 W at <0.1% THD+N into 4 Ohm, 20-20,000 Hz. Also, "before distortion kicks in", probably should be more specific too, such as "before distortions start to rise rapidly...." something like that..
Dynamics relates transient peaks and dips - speed and accuracy of following a momentary transient in the music - this is determined by the transformers and capacitors within an amp, even at the same wattage level.
I would add that, it also relates to the rated slew rate (typically not specified for AVRs and often not mentioned in entry level separates), and the output devices. If you compare output devices of two 100 W amp, one may have the same output device, but use two in parallel. That would obviously enhance the dynamic peak capability depending on how the power supplies match up with the two compared units.
Well designed amps (tube or not) will have great dynamics resulting in better enjoyment of the music even at low/moderate levels. Demanding speakers (4 or 6 ohm) require better (not necessarily more watts) amps to be able to deliver these large spikes in current.
That's sort of an almost always true general statement that no one will/should argue.
Low/mid range avr’s and amps in desktop style amps like the Sonos will sound relatively flat, even if you can turn up the volume high.
This is not always true because it depends on the applications. If such "desktop style" amps are in fact designed for desktop use and if use that way, it may be able to sound very dynamic when used well below its rated output. Example: In my desktop system, I use two satellite style speakers and a passive subwoofer. My 50 W/4 ohm rated NAD amp sounds like it has no dynamic limits at all, and in that setup even my 5 W DIY amp would sound just as dynamic. So it all depends on one's need for "watts", that's why one should always use a good spl calculator to figure out the need, then take the number, multiply it by 200 to 300 (better obviously) and then forget about the lack of dynamic concerns.
It used to be that quality of an amp was determined by weight (not watts) - good transformers are heavy. However there are now several switched mode power supplies and class D amps that can produce great dynamics without the weight - they aren’t cheap though.
I wish more people would make this same point. Again, just to add though, modern day transformers could have better core material and wrapped in ways that the output to weight ratio could be much improved. As well, different heat sink material could also result in better performance to weight ratio, plus the use of thermal controlled quiet fans, one amp can have much better output to weight ratio. Just compared a 300 W Bryston or Anthem MCA amp to a 300 W Emotival, or Monolith class AB amp, one will see the difference immediately.
I know you know your stuff, I am just adding a little more specifics for others to consider..
