Hi Blasto,
You have asked, I think, some very relevant questions. I also do have very similar questions in my mind and of late have bored people to death by asking very similar questions in my very long amp thread.
I can try to explain my current level of understanding at this point.
Let's first clarify one thing: the 40 Watts or 50 Watts that you are talking about in your post are what is called the rms power continuously deliverable at a given speaker load (usually given for 8 Ohms). However, the continuously deliverable rms rating is NOT the maximum power that the amp is capable of delivering. A well-constructed amp may be able deliver for a very short time period an amount of power much much higher than that power rating. This is very important for music reproduction, because in music there are quieter periods, and then there are sudden bursts of energy. Because of this the above mentioned power-reserves, called dynamic headroom, are a must.
The other aspect is the amount of power that the amp can deliver for different speaker-loads. As the music flows, it involves different frequency components at different times. The speaker impedances change with frequency. That actually means that the amp sees the speakers as loads where the impedance is changing continuously with time. Now from the Physics formula relating power, voltage and impedance you would discover that as the load-impedance is lowered the power requirement is higher if you keep the voltage fixed. So you see, as the music is being played, the amp is doing a tremendously non-trivial job of delivering an expected level of power to a load where the impedance is continuously changing.
That's why more than the speaker sensitivity, its impedance as a a function of frequency (this in general depends also on the power) is an important aspect. On the amplifier side, some of the things to look for is the available power as the impedance changes and also as the frequency changes.
Now you can guess what would be the ideal situation. For the speaker, the ideal situation should be the impedance completely independent of frequency (at least within the relevant range of 20 - 20KHz). This is usually called the flat response because an impedance vs frequency plot would look perfectly parallel to the frequency-axis of the plot.
The ideal amp (at least with respect to the things we are discussing here) would be one which would exactly compensate for a reduction of speaker-impedance. For example, a 50 wpc@8 Ohm amp would become a 100 wpc@4 Ohm or a 200 wpc@2 Ohms or 400 Watts@1 Ohm. BTW, this does not usually happen for low-priced amps.
Also, ideally, at a given speaker impedance, the amp should deliver power independently of the frequency.
Obviously, there are many many other aspects for the amp (and also for the speakers), especially with respect to distortions. The THD rating given as an amp spec. is also very misleading. Two amps having the same THD can have very different distortion patterns. In addition, distortions in lower harmonics are generally considered better. Also odd and even harmonics make a difference. To top it all, all these can change at different power.
So after all this mumbo jumbo, what is the conclusion? What we common people should do?
To answer one of your questions, yes, a 50 Wpc@8 Ohms amp should be enough for all kinds of listening at home, even for large rooms and at deafening volumes (look at my table in a recent post in my amp thread), provided the amp is close to having the ideal characteristics as described above and also having enough dynamic headroom to accommodate a sudden 15db or so (at a max, even good recordings usually do not have this kind of dynamic range) volume increase.
Obviously in the budget segment, you do not often find an amp of 50 wpc@8 Ohm to have even close to the nice characteristics described above. But this discussion should be a pointer to a few things. For example you would notice that in the market there are some 50 wpc amps at less than 20K and some other 50 wpc amp at 40 or 50 K. There are even 25 wpc amp at close to a lakh. Obviously some of the price difference can be due to business-related economics, but in general there IS a difference.
At least for SS (solid state) amps, that's why people want to make sure that at least some of the ideal characteristics are somewhat approached by buying an amp having a power rating much higher than the actual requirement.
So, if all the ideal characteristics are met including the one for dynamic headroom, there should not be any difference of sound between the 40 wpc Nad 515 and 50 wpc Nad 525 producing the same volume at the same speaker-set (assuming the two amps are 'voiced' the same way). Now based on the above discussion, you know that there is enough reason to believe that there can be many other significant differences between the two amps which can make a real difference in the sound produced by the two amps.
hyeah:
