dsper, with the CS 5's it's the amps capability into 2 Ohms that is most relevant. I would only consider amps that could double down and down again into that load. I would recommend by starting with amps spec'd with the capability to deliver at least 400 Watts into 2 Ohms, and ideally 800 to 1600 Watts into 2 Ohms. After that it comes down to the sonic characteristics of the amps under consideration and personal preference.
These speakers amp requirements is perhaps the only reason I don't own them. Driven properly, they are wonderful!
Krell ;-).