Speaker impedance is not dependent on signal level.

To your other question, the concern is the amplifier's ability to handle transient peaks. THD measurements are measured using a continuous tone.

Backing up to the beginning, what prompted you to think you need a new power amp?  From reading the discussion I'm under the impression you don't need one.  Rather, it sounds like you've been sucked into the idea that you will achieve audio bliss by buying a new amp from reading things on the web by sources that are highly questionable

Quote from: Rich B on 2014-01-01 20:54:28

Could you explain what you mean by "it is the same whether the music is loud or soft"? I'm not entirely sure what you mean in your second sentence. English is not my first language, so I'm trying to understand this as best I can.

Let me try. We are talking about four different things here - power, voltage, current and impedance. They are all related. Power is what counts, and is the result of voltage multiplied by current (P = U * I), so a result of both voltage and current. Current depends on voltage and impedance (this is Ohm's Law), current is voltage divided by impedance (I = U / Z). If you combine the equations, you get P = U * I = U * (U / Z) = U²/Z - thus power depends on the square of the voltage divided by the impedance. The impedance doesn't depend on power, voltage or current, but varies with frequency in a complex way. It is usually specified as a "typical" or "average" value over the frequency range, or at some specific frequency (so typically 4 or 8 ohm).

Okay, but watts are watts? Volts are volts? You don't get better quality current? Or is it all the same?

You mentioned impedance does not depend on power or current. But I thought low impedances required more current? My speakers have a nominal impedance of 8 ohms but it dips down to 3.4 ohms.

Re: driving your speakers to their fullest extent
Is your system not loud enough for you?  Do you hear distortion when it is turned up to be as loud as you'd ever listen?

The answers to these questions are far more important than understanding the relationship between voltage, current, impedance and power.

Quote from: Rich B on 2014-01-01 22:23:44

Okay, but watts are watts? Volts are volts? You don't get better quality current? Or is it all the same?

Not all devices are of equal quality.  These calculations are just to figure out if a given device is able to drive a spaker without being overloaded.  It may or may not provide good quality audio (although generally most amps aren't bad if used within their design load).

So there is such a thing as good quality wattage and bad quality wattage?

If amps are not being driven beyond their limits would the quality of wattage still matter?

Quote from: Rich B on 2014-01-02 12:47:11

So there is such a thing as good quality wattage and bad quality wattage?

Not really.

So something like this :

If you have an audio editor you can easily produce a piece of music with any degree of clipping you want simply by amplify it different amounts beyond 0dBfs (before playing it). Once the signal is clipped, it will be clipped no matter how softly you play it.

While this will be clipping, and will sound bad if you take it very far, it will not sound quite the same as clipping produced by driving a high power amplifier beyond its limits and especially by driving speakers beyond their limits.

Playing a clipped signal at low to moderate volume will not hurt anything, except maybe your aesthetic senses, but actually driving the amplifier or speakers into very much clipping can be very hard on them.

By hard on them, I mean blowing out the output transistors or damaging the speaker coils.

How much gain +- could I expect from the room that the SPL calculator would not add to the figures? 6 dB? 8 dB? Just a ballpark figure.

I think the only question that remains for me is what clipping would sound like.

I know if I listen to music with VLC player and turn the volume in VLC to 140% the sound becomes distorted, especially during peaks while wearing headphones. Is that what I could expect while an amp clips?

If I remember correctly it was 15 watts into 8 ohms. Which means 30 watts into 4 ohms and 45 watts into 3 ohms (speakers have a minimum 3.4ohm dip according to the B&W spec sheet).

Quote from: Rich B on 2014-01-03 10:09:55

If I remember correctly it was 15 watts into 8 ohms. Which means 30 watts into 4 ohms and 45 watts into 3 ohms (speakers have a minimum 3.4ohm dip according to the B&W spec sheet).

Those look like peak figures to me and the impedance should not be a factor.  What is the crest factor?  What is the distance?  What is the efficiency of the speaker?

I've been extremely satisfied with a 60W Yamaha RX-460 in a small to medium-sized room since I bought it in 1994.

Wikipedia should tell you all you want to know about crest factor.  I am also curious as to why you are providing such vastly different power levels.  If I wanted to know how much voltage or current was needed to drive your speaker at a specific frequency it would matter but I was only interested in power.  I was asking you to consider how much power was required for typical nominal listening levels in order to get a better feeling for what this stuff really means.

I'm glad to see that you now realize that you don't need a new amplifier.

EDIT:
BTW, my 60W receiver?
It's 60W @6 Ohms. At 8 Ohms it's rated @55W.
If I were to use the amp to drive a 6 ohm speaker at 90 dBSPL @1m with a 1kHz tone, it would deliver the same power as if it were driving an 8 Ohm speaker at 90 dBSPL @1m with a 1kHz tone, provided the speakers had the same efficiency.