efficiency- hope i spelt that wright.

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.