Hello Tvad
I am using Cabasse Catalane speakers, two 8 inch woofers in diapolito configuration in a transmision line, concentric midrange and tweeter unit...amazing sounding, a devil to break in, much worst break in than lowthers but for a shorter time...muuuch more detailed than lowthers also.
Yes they go down to 30 hz no problem efficiency is 94 db and yes 4 ohms: I use 12 watt push pull class A Leak monos and they bring down the house, I was using an 845 SET 22 watts and they ran perfectly (I like PP better I guess).
If the speaker manufacturer only uses the normal available drivers like SEAS, vifa, peerless etc, yes no small foot print will give you good efficiency, but if they make their own drivers....you know.