Speaker Efficiency

@paradisecom  Looking at all the great information provided above, if your 6watt amp can provide 70db with 85db speakers, and your not on the higher end of the volume control, your probably not taxing the amp too badly. Continuing to play at louder and louder volumes, would create more distortion, and sound quality would diminish. Higher efficiency speakers would work better with the Decware amp, and should have less distortion on the signal, and overall could sound better.  If this is an office system, you probably don't need 40 watts.

I'm not sure whats out there for high efficiency bookshelf speakers, but if your system is in your smaller office, you may not need to be in the mid to high 90db range.

These are interesting, albeit expensive

https://devorefidelity.com/devore-fidelity-speakers/orangutan-series/micr-o-speakers/

 I have been listening to my Decware 300b tube amp for a few months now. At 8 watts, I have them paired with Klipsch Forte IV at 99db efficiency, and this set up has been sounding great. This morning I connected a pair of Sonus Faber Electa Amator speakers, 6ohms at 88db of efficiency. They sound fantastic, with more weight  and depth. I understand there is a huge price difference in the two speakers, but aside from that, if I am getting 80db of volume at my chair, 8' away, what is the draw back of using lower efficiency speakers with SET amps, so long as the volume is sufficient? 

It comes down to your choice of music as well. Here.. listen to this track by Icelandic composer, Hildur Guðnadóttir to check bass control and so on of your drivers. Turn subs off and just crank the speaker up to about ~90 db (Don't be shy, our good lady, Hildur is merely here to check for a minute if your stuff is any good or not). Compare back to back with a pair of Schiit Tyr Monoblocks and the Decware weak sauce fleawatt on the Electa Amator. When the Decware cries and runs to its daddy, i.e., fizzles out, i predict that i will see it on the Audiogon marketplace for sale real quick. 😁

https://www.youtube.com/watch?v=B7sj_XJJ9U8

But, If you are listening to some weak sauce track with a  twangy dude wailing to a single string at 65, 70db about how a gal broke his heart into just a few million pieces..... i suppose your Decware should be just fine with the Electa Amator.

@deep_333 Funny, :)

Ok, I did just that. Hildur Guðnadóttir 12’ away, at 85db, the "fleawatt" was at about 75%, the Roon app was at 80%, if I had to guess, I was probably running at 2, 2.5 watts. My room is 14X20 with vaulted ceiling. I didn’t hear any problems with the signal, although the room was shaking, and the dog ran like hell :0. But the real question is, why? You cant honestly listen to that soundtrack at 90db and enjoy it. I do like Schiit stuff, I’ve had Yiggi and Bitfrost DACs, I dont need a 200 watt Schiit Tyr to drive a pair of Forte’s? I do have a Hegel H590 (2 x 301 W into 8 Ω, Dual Mono) for the Sonus Fabers, maybe I’ll do a comparison.

@marktheshark wrote:

I have been listening to my Decware 300b tube amp for a few months now. At 8 watts, I have them paired with Klipsch Forte IV at 99db efficiency, and this set up has been sounding great. This morning I connected a pair of Sonus Faber Electa Amator speakers, 6ohms at 88db of efficiency. They sound fantastic, with more weight  and depth. I understand there is a huge price difference in the two speakers, but aside from that, if I am getting 80db of volume at my chair, 8' away, what is the draw back of using lower efficiency speakers with SET amps, so long as the volume is sufficient? I did notice turn the Pre amp, the tube amp and the Roon volume towards the highest setting, it starts sounding terrible....

There's the pragmatic approach that holds that if sounds great (i.e.: better) to your ears with an 8 watt SET amp to 88dB sensitive speakers in most any situation you can throw at it, then that's all that matters. 

I'd tempt that approach though by saying that such an amp/speaker combo defeats the purpose of the SET in particular; such an amp, to be its best, should be used using only a fraction of its output power to take advantage of its low distortion here, and for that you need high efficiency speakers (i.e.: no lower than honest ~98dB's) - preferably high impedance load. I would also be cautious about your 80dB listening level number as an indication of the actual load presented to the SET amp - at the listening distance, that is, with the particular speakers. You may well be running the amp closer to its ceiling than you might believe. 

Speaker efficiency is sensitivity + impedance vs phase angles.

It is not solely the sensitivity spec, which is why you can’t assume a 90dB-sensitive speaker will mate well with a moderate/low power tube amp. This is why you don’t often see Wilson or Magico owners using tube power. 

Low impedance drivers generally perform better than their high impedance counterparts. They require more from the amplifier, but if the amp is up to the task, the speakers will perform better and have greater dynamic range. This is why the large ultra-high-end speakers in the $30K^ range almost always have a low average impedance. The only exceptions might be those with unconventional bass loading and compression/horn drivers. This isn’t to say all speakers with a low nominal impedance are “high-end.” Many low-nominal speakers have 8-ohm drivers, but the combination and number of drivers coupled with complex crossovers lowers the speaker’s average impedance. This is why it’s not uncommon for a <$4K tower speaker to have a 4 or 6 ohm nominal rating when in fact all its drivers (excluding the tweeters) are 8-ohm. 
 

All but very few speakers have impedance swings that will result in non-linearity when coupled with tube amps. The exceptions are rare. One reason tube amps are so popular is because the typical speaker load will result in a boost of the midrange and upper bass frequencies. It makes them louder and thus often results in the false perception of greater resolution and power delivery. It’s why you’ll encounter comments on the forums from Joe Schmo who claims his 20 WPC tube amp sounds more powerful than his 100 WPC solid state amp.  The reality is 20 watts is 20 watts, and Joe just hasn’t cranked the volume quite enough for the compression to become obvious. And since tube amps “soft clip,” when overdriven, the sound flattens before it becomes obviously distorted.