How important is the efficiency of a speaker to you?

I think a very reasonable argument can be made for low-power amps, along the lines of “the less you mess with the signal, the better.” So assuming you select a well-designed amp, you simply find a speaker that pairs well with it. I have a couple of Pass diy amps with 15 to 25 watts. I’ll call that low power in today’s market. So to go with the 15-watt amp (at 8 ohms) I built speakers with 95db efficiency. Nice sound, really sweet midrange in a 20x20 family room. The 25-watt amp is a little more flexible. It makes 40 watts at 4 ohms. And I like Magnepans, but they’re 86 db and 4 ohms. I tried it and liked it. Volume probably tops out in the low 90s db range in the large room they’re in, but it’s a living room, not my music room, so fine by me. I’ll add that among the high efficiency speakers I have hear at stores and with my local audio group, they have a dynamism I find appealing, really lively. But I value timbre above all, and that usually leads me to Maggies.

High sensitivity speakers w/ true bass extension necessitates large volume cabinets which can mean more $ & potential room / aesthetic placement challenges. That said, as John Atkinson often points out, high sensitivity drivers have substantially less distortion at higher output levels because of the much lower current flowing through their voice coils which generates much less heat & are “ working” much less hard. That said, some may not like the sound of the driver type used to create that high sensitivity ( horns, AMT’s etc). Every method has trade offs.

My own personal preference that to me sounds closest to live music is high sensitivity horns w/ tube amps but not “ flea powered” if you like to listen up loudly in a big room. Very difficult for just about any  lower sensitivity speaker at any price w/ even w/ heroic amplification to match the speed & instant dynamics of a quality horn loaded one. 
 

Volti Audio Rivals do it for me….

@ronboco 

As far at cost to achieve SET sound with power I have no idea.  If you're are really interested you should reach out to somebody like Pat Hickman at Whammerdyne.  He builds both "flea watt" and hi power hybrid amps and I'm sure he could explain both the benefits and drawbacks of the different levels of power.

Not very important. 

The best speakers I have ever heard, and the best I have ever owned (not in the same set, I could not afford the best I've heard), have all not been very efficient.

My old Koss Model 1a full range electrostatic speakers were 87db. But the clarity, detail, attack/decay, soundstage and image they produced are better than pretty much any high efficiency speaker I've heard. 

 I can think of a few exceptions, like the Acapella Audio Arts Hyperion, with their plasma tweeter, and their built in amps for the woofer section. 

Not particularly important to me.