I think they're 8 ohm, don't recall exactly. Danny Ritchie (the designer) says the're pretty efficient and will work well with amps of 50 Wpc or better. A line array has a higher perceived "signal to noise" ratio than more conventional topologies; they're sound louder at a given measured volume than a normal 2 driver 2 way.
You can get more details on them at AV123's own website or at the GR Research forum at AC.