I googled ATC 7, and it looks like the sensitivity is rated for 84dB @ 1W @ 1m. Yeah they are rather inefficient. I used to own Revel M20's with 87dB sensitivity, and they were fine with Classe CAP-151 and Krell 300iL integrated amps. I think you need at least 100W clean power for your speakers, but keep in mind quality is more important than quantity.
