Well, I just bought fairly efficient Sonata III loudspeakers (92dB, 8 ohms), and they STILL require copious power to control the bass for rock music...as stated by Alan Yun to me in a telephone conversation.
Here I thought I was going to be able to drive them with a low power tube amp. Nope.
After this experience, I've given up trying to match amplifiers to speakers based on the specifications.
I'd suggest going directly to the manufacturer of your loudspeakers and pointedly asking what they recommend for amplification based on your listening preferences and room. If they don't have a recommendation, then buy a different brand of loudspeakers.
There is a lot more to matching efficient speakers & low wattage speakers. An example is impedance. Just ask anyone with a single-driver loudspeaker.
My Wilson Watt Puppy 7's have 92 dB efficiency, although at 4 ohms impedance they draw twice the current that one rated at 8 ohms would have. I tried some 15 wpc SET amps with them, and while they would drive them to reasonably satisfying sound levels, the bass was loose and dynamics were compressed. 92 dB is not hugely sensitive, and the impedance curve also plays a big factor. If your speakers have impedance dips and/or the impedance is reactive (either capacitive or inductive), the speaker load may be such that it would tax your low-powered amp. As Tvad says above, I would contact the manufacturer for an impedance curve and recommendation on the types of amps you can drive them with. Also, low-wattage amps are not always cheaper--there are a lot of factors in cost--and they may or may not be a good match for your speakers. For instance, Lamm amps that have 18 wpc cost something like $30K and are raved about here on the Gon.
There are a few things you must do on paper when it comes to Amp/Speaker combinations.
1. Find out the lowest impedence of your speaker. If lowest impedence is 4 Ohms you should be fine, anything lower than 3 Ohms, you need to make sure your amp can handle this otherwise it will clip i.e run out of power.
2. Find out the minimum power required by the speaker based on its nominal impedence. If the specs say 30-250W @ 8 Ohms, you need a 30W amp and above e.g 60W would be fine. Can you use a 10W amp? Yes but you risk damage if the speaker demands the extra power and the amp clips trying to deliver and in some cases blow the tweeters as well. If no minumum is specified you have even use a 3W SET.
3. Efficiency is only a factor if you have a large room e.g 16x20 and/or you want to listen loud. SPL drops 6dB with each doubling of distance. So if your speaker is 87dB@1M and you are sitting say 3M (10 ft away) and your amp was using its first watt your SPL would be around 78dB. So what is loud? 85db is loud and safe. 90db is very loud only safe up to 8 hours per day before hearing damage occurs.
4. How much power then? Each doubling of power produces and increase of 3dB. So as per example above, you would need 78dB(1W),81dB(2W),84dB(4W),87dB(8W).
After all this homework, you should now listen to that amp/speaker combination.
BTW, my own speakers are rated at 87dB/M but specify needing an amp between 100-400W @ 4 Ohms, I am using a an capable of 500W @ 4 Ohms. On paper this amp will never cough up anything more than 6W at the volumes I listen to and only on demanding material would the amp ever have to call on its reserves.
Is a guy wasting his money buying inefficient speakers?
No. There is a lot more to speakers than this. Distortion. Dispersion. Driver integration. Compression. I have heard 83 db sensitivity speakers that play much louder than 91 db sensitivity speakers even if you drive the 91 db speakers with the same powerful amp that the 83 db speakers demand....a lot depends on Xmax and thermal compression. Many high sensitivity speakers with very light weight cones and long voice coils in small magnetic gaps (aka cheap drivers) will distort and compress very quickly.
"the lower the wattage, the cheaper the amp" is way off line. I'd take a 7 watt 300B tube amp over a 200 watt mass production solid state amp any day. The 300B is probably more money too. Quality, not quantity!
In addition to what's stated above about efficiency and impedance speakers with exceedingly complex crossover design, i.e. Wilson, will be intrinsically harder to drive no matter how forgiving the other specs. That's why speakers such as Reference 3A are so easy to drive with low-power amps, the crossover is a single capacitor, so power transfer is ultra efficient.
I agree with Elvick...quality first, however, if you do go with a mere 7 watts then you definitely need a highly efficient speaker...an 83 db speaker will be severely restricted if driven by 7 watts....in that case you might consider horns as an excellent match.
Magnus, a speaker should always be viewed in relation to what amplifier it would be used with.
If you have a low output (low wattage) amplifier, especially the vacuum tubed variety, you would want to use a fairly efficient speaker with high(ish) impedance rating and benign impedance curve.
If you have a huge, high watt solid state amplifier, you would normally want a less efficient speaker with a lower impedance rating. Very efficient speakers on big amps sometimes result in a audible hiss. (Note, I said sometimes, not always). Low efficiency speakers go a long way to hide amplifier and line noise.
Youll find that many very efficient speaker are expensive, and it normally works out cheaper to purchase a bigger amp with less costly less efficient speakers.
Low output amps are also not always cheaper. Many single digit output tube amps can retail for many tens of thousands of dollars. Price of the amp is determined by a number of factors, not always simply on output.
Have 94db Horn with 7" coil drivers.I tried some SET and it will work but again as mentioned above you often have to discern what they will play with and what they will require to open up and give dynamics's.Fro mine I would say that only a 22 wpc 845 tube SET has enough power or better a 40 watt push pull.And as I have noticed many speakers require more than specs say.B&W's I sold (say the 803 at 8ohm nominal and 90db sens you would see that and think it's an easy drive.But try that speaker with a 80-100 SS or a 45 watt glass amp.It had such a nasty impedance swing on transients a t volume that would be cheating youself with less than 200 watts SS 150 glass better a 300 watt SS and 200 tube amp though they would work with 100 tubes just not open up.And they were speaker that did not get much from tubes as many others.As always few specs can be looked a without taking ion many others to judge what there requirements will be.
Some of the issues regarding amplifier vs speaker selection are discussed here:http://www.atma-sphere.com/papers/paradigm_paper2.html
My own feeling is that if the efficiency of the speaker does not interfere with the revealing nature of the speaker, does not interfere with bandwidth, then the efficiency is valuable! My own speakers are 97db 1 watt/1 meter, compared to a speaker of 87 db, the result is that I can run an amplifier of 1/10th the power and get the same sound pressure. Fortunately, one thing high efficiency speakers do really well is dynamics- no worries there.