Of course with efficiency inversely goes audio output 'volume' and subsequently battery life - headphones don't care too much about that because of the intimate coupling of the speaker to the ear and therefore not much 'volume' is needed. For a little desktop, portable speaker the issue is different. Lots of speaker designs out there that are small however, but tons of research goes in to making them efficient enough and deliver good audio output. Maybe they are worth a try at only around $30 for the electronics and speaker, or maybe there is a 'better' speaker that could be substituted. Of course, the the amplifier needs to be actually capable of delivering a full spectrum response with enough energy to drive the low end, and we are talking about a pretty small battery. - Thanks Matt for the idea!