I just last weekend heard Bob Brines' products running with a 8-9 watt SET and a 6 watt T-amp. They are for the most part Mass Loaded Transmission lines so not quite as efficient as a back loaded horn but also without many of the disadvantages of a back loaded horn. The Fostex drivers are medium high sensitivity and the Lowther is outstanding if it suits your priorities.

http://www.geocities.com/rbrines1/

"Brines Acoustics"

A horn type speaker, folded or otherwise, provides great efficency. But the problem with using a horn speaker is that for a horn loaded speaker to cover the lowest frequencies it has to be huge. I mean really huge. Most commertial horn speakers have a low frequency cutoff that is not all that low in frequency. It's always something.