Another way to go for a large room is to use horn-loaded speakers for maximum speaker efficiency. Not that you are going to spend $10,000 a pair for speakers, but just for kicks check out Avantgarde's web site. (
www.avantgarde-usa.com). I think all of their speakers put out SPLs in excess of 100 dB with a 1 watt input, so 1 W into an Avantgarde speaker is like 100W into a "normal", decently efficient speaker. In their pictures, the horn that looks like the bell of a tuba is actually the mid-range. Their "smaller" speakers use conventional subwoofers in the base, but they also make a "Basshorn" that is a horn-loaded subwoofer that occupies a lot of floorspace, to say the least!
Avantgarde's speakers have gotten a lot of rave reviews in Sterophile. I think the people who buy them are the same ones who buy 5 Wpc single-ended triode tube amps, costing many thousands of dollars.
A more practical approach is to get a set of Klipschhorns, or an old pair of Altec-Lansing "Voice of the Theaters". Both use horn-loading not just for the tweeter, but also for the woofer. As a result, they are both huge boxes, but they are also both very efficient, probably pushing the 100 dB SPL level. Keep in mind that these speakers were designed to fill a commercial movie theater with sound, back when all they had for amplifers were low power tube amps. (certainly well under 100 W).
For that matter, I have an old Victrola that has a horn-loaded cabinet. Volume control is by opening/closing a pair of doors to the wooden horn structure. It provides pretty strong volume from the needle's vibrations, with 0 watts per (one) channel.