Same watts at 8 and 4 ohms?

If your amp puts out 150 watts into 8 ohms and 200 into 4 ohms, it will drive your speakers just fine if you only need 100 watts into the 4 ohm frequencies to drive them as loud as you want to listen to them.

Is that right, @atmasphere ?

It is- and more to the point, in this example above, even though the amp doesn't double power **at full power** into 4 ohms, we can easily see that it does increase. If this amp has feedback, its output impedance will be low enough to enable it to act as a voltage source- all the way up to its rated power into 4 ohms. So what if that isn't double the power of the 8 ohm rating?? We are only talking about the amp at full power. At any other power level it will be just fine, meaning that it will indeed double power as the load impedance is halved. This holds true for impedances of less than 4 ohms too- but one must be increasingly aware of the limitations of the amp when coupled to such loads. As long as the load is easy to drive (such as Wilson speakers, which are easy despite their low impedance in the bass) it won't be an issue.


Now someone touched on a different issue- that lower impedance loads cause amps to make more distortion. This is true, and is why you really don't want to make an amplifier (especially a solid state amp) work hard for a living! When it makes more distortion, especially if solid state, most of that will be higher ordered harmonics and IM distortion, both to which the ear has a keen sensitivity! IOW if **high quality audio** is your goal, regardless of your amplifier, a higher impedance load will do better justice to your amplifier investment. OTOH if **sound pressure** is your goal, you have that slight 3dB increase in volume if your speaker is half the impedance. IME the latter isn't worth it, but I like the stereo to sound nice :)

The distortion vs. output profile is a little more complicated than explained earlier. Please see figures 5 and 7 for what a very well behaved linear amp’s output usually looks like vs. output power.


https://www.stereophile.com/content/luxman-l-509x-integrated-amplifier-measurements


The limiting factor is often notch or crossover distortion, which is constant and occurs near zero output. As the output rises this distortion becomes a smaller proportion. Of course, eventually the amplifier will clip and distortion is dominated by the top of the waveform being flattened as the voltage at the output reaches the voltage of the power rails.
Of course I am a Luxman fanboy but this curve is very common.

Okay Guys, 

Not to steal the OP's thread but I have what I think is a similar question in the spirit of the thread. 

My main speakers are Thiel CS5's and we have talked on other threads  about how they are a difficult load as they dip close to 2 ohms and 82 dbl sensitivity. There has also been associated discussion about how more power can mean more noise.

I have been thinking about the idea of less power creating less distortion and the ability to hear more detail.

Assuming my last sentence has some validity, and wanting to try something a bit different, I am taking delivery on a pair of speakers rated at 8 ohms and 88 dbl sensitivity that I do not believe dip below 6 ohms.

I probably do not listen to most music louder than say 91 dbl at about 9 feet. 

So... what watts do I need to be happy with a good tube amp? I have been hallucinating that a tube amp with a good 25 watts into 8 ohms and that holds 25 watts into 4 ohms should work quite well as a place to start.

Am I anywhere near to correct about this? What else should I consider?

My apologies to the OP if I am too off topic here.

Thanks for listening,

Dsper

Every time a question like this comes up it reminds me what a good decision I made way back in the 1970’s to avoid hard to drive speakers with low sensitivity. It just makes life so much easier. Doesn’t hurt that every hard to drive low sensitivity speaker I ever heard never sounded that good to me. Maybe because they were never used with an amp that could make them sound good? That could be it. But whatever, who cares, not my problem!

It was only much later on that I discovered tube watts and solid state watts are nowhere near the same. People can howl all they want, throw all the technical mumbo jumbo they want, when you hear 50 tube watts have more authority than 200 SS watts you have to admit all the words in the world can’t change the fact tubes just plain sound way more powerful than they measure.

Put it all together and you blew it once, and are blowing it again. Just not by as much this time. But its no hallucination. If anything stands a chance of making those speakers sound good its a good tube amp. I drove Talon Khorus for 15 years with 50-60 watt tube amps, and they went a lot louder than you're talking. So you will probably be fine.