When talking about low powered amps and high efficiency, it's all relative. 18 W is not really that low powered and you have a lot more options. Personally, I prefer ultra high efficiency speakers no matter how much power. When I mean high efficiency, that is certainly 97 dB or higher.
Based on your conditions, there are many possibilities. I would recommend a speaker built around a PHY-HP driver. Check out either Ocellia, Musical Affairs, Tonian Labs.
http://www.sorasound.com/musical.htm
http://www.sorasound.com/musical.htm
http://www.tonianlabs.com/tonianacoustic.htm
These are 97 dB and have deep bass. I wouldn't rule out horns entirely. Depending on what you consider huge, you can check out several Altec/GPa 604 based speakers. I use a 604 based speaker but my speakers are huge.
http://www.jean-hiraga.com/
http://www2.117.ne.jp/~y-s/speaker.html
There are also a bunch of back loaded horn speakers that are fairly compact and conventional looking. The Feastrex based Maxxhorn speaker is pretty cool.
http://www.lovecraftdesigns.com/index.html (formerly Cain and Cain)
http://www.maxxhorn.com/index.php?option=com_content&task=view&id=54&Itemid=256
Along more conventional lines, the speakers mentioned above like Coincident, Devore, etc. would all probably work. Many of those are not really high efficiency but with 18 W many would work just fine. Those speakers would require more careful matching in terms of power since it is borderline. The speakers I suggested would all work in terms of power and it would be more of an issue of synergy with your amp and personal preferences.
Based on your conditions, there are many possibilities. I would recommend a speaker built around a PHY-HP driver. Check out either Ocellia, Musical Affairs, Tonian Labs.
http://www.sorasound.com/musical.htm
http://www.sorasound.com/musical.htm
http://www.tonianlabs.com/tonianacoustic.htm
These are 97 dB and have deep bass. I wouldn't rule out horns entirely. Depending on what you consider huge, you can check out several Altec/GPa 604 based speakers. I use a 604 based speaker but my speakers are huge.
http://www.jean-hiraga.com/
http://www2.117.ne.jp/~y-s/speaker.html
There are also a bunch of back loaded horn speakers that are fairly compact and conventional looking. The Feastrex based Maxxhorn speaker is pretty cool.
http://www.lovecraftdesigns.com/index.html (formerly Cain and Cain)
http://www.maxxhorn.com/index.php?option=com_content&task=view&id=54&Itemid=256
Along more conventional lines, the speakers mentioned above like Coincident, Devore, etc. would all probably work. Many of those are not really high efficiency but with 18 W many would work just fine. Those speakers would require more careful matching in terms of power since it is borderline. The speakers I suggested would all work in terms of power and it would be more of an issue of synergy with your amp and personal preferences.