I think this FAQ from Legendary Audio Classics is very helpful. It explains why the Sony 20 watt amp I talked about earlier sounds so dynamic:
Why does a 35 watt Marantz sound better and louder than many 100 watt receivers?
A: Because 35 watts/channel as specified by Marantz in the 1970's meant...
" The unit can deliver 35 watts into 8 ohms for one hour, from all channels at the same time, with no significant change in distortion, or other specifications, at any time during, or after, the test hour."
...while 100 watts/channel today (for instance, in my JVC surround system) means...
" The unit can deliver 100 watts for a fraction of a second, in one channel only, if the other channels aren't running and nothing else high energy has happened to drain the power supply of stored energy in the last few seconds."
In fact, my JVC 5-channel Dolby surround receiver claims 500 watts RMS, but the power consumption label on the back panel tells the story:
If my JVC receiver was 100% efficient, meaning that every bit of power it took from the wall was delivered to the speakers as audio power (which it isn't), that'd give you only 64 watts a channel, about 2/3rds of the claimed power rating (which is 100 watts per channel, remember, 500 delivered as 100 per each of the five channels.)
But since the receiver can only (at best) convert about 50% of the available energy to the speakers, and the available energy is what is left over after the heat is generated (did I mention that this model JVC runs almost too hot to touch on top, even when making no sound at all?) and the watts that go to lighting the panel and powering all of the other circuitry are accounted for, the system can perhaps, when brand new, on a good day, generate 32 watts a channel continuously with all the channels going, which is pretty sorry compared to the claimed 100 watt per channel rating. That is less power per channel than an old 2235 receiver. Shocking, eh?
Turning it around, because of the way that the units were rated in the 1970's, that classic 2235 Marantz receiver, rated at 35 watts a channel, can dependably produce much more than 35 watts in both channels at the same time for a minute or two (far longer than the peaks in a modern receiver.) An honest rating for use with music for the power amplifiers of an older Marantz is generally in the range of 120% of rated power or even higher.
These ratings were instituted because of many false claims for power output that were being made using many different types of power measurement and general baloney at that time. IHF, RMS, Peak, Peak Music Power, Average, etc. RMS is what was settled on, and it's still widely used today, but the one hour rating was dropped some time back.
Interestingly, the situation that caused the RMS for one hour ratings to be made standard is now recurring - as I mentioned above, my JVC's ratings are pretty obviously designed to deceive the consumer to an extreme degree. Certainly there is no way that they can claim that those ratings paint an accurate picture of the amount of power the receiver can actually deliver in real world conditions - loud music and cinema surround takes a lot of power, in a lot of channels. Try listening to Jurassic Park... wait till the Tyrannosaur walks up behind you, or there is something exciting going on. Those 32 watts are pretty puny...
To the relatively straightforward power issue, you can add the fact that the design of the audio and RF circuitry in a Marantz is absolutely top-notch, and you can hear that in the character of what little distortion there is, in the way the bass, midrange and treble controls (and loudness contour and filters) affect the signals, in the way the FM signals come out sweet and clean, and so on. As an engineer, I really don't like to drop into using descriptive terms meant for food or lovemaking and so on for sound, but you know, when you A:B a Marantz against other units that are supposedly equivalent, the bottom line is it sounds better, and obviously so.