I'm using it in a HT setup with an SR5007 rated at 100W per channel. In 2 channel mode I imagine it gets close to that rating and have reached my tolerance for 'loud' with them. They're 97 db sensitive which is really good. Not quite reference level sensitivity but even lesser amps should be able to get them loud.
Marantz SR-5007 Klipsch WF-35, RP-450C, WB-14 theater
Marantz 2325 Klipsch Forte ii stereo
Sony PS3, Xbox 360, Wii U