A low powered push/pull tube amplifier; something like a 2a3 or 300b or 45 based amp making between 4 and 20 watts. SET amps can make a bit of noise that highly sensitive speakers such as Klipsch can amplify easily. Push/pull amps, correctly implemented, do not have nearly the noise issues of SET amps.
I think I read somewhere that a little 5 watt push/pull amp is what Paul Klipsch designed the horns to work with but I would not put that in the bank. This is where a very high quality low powered amp (think Wavelength...) comes into play. More than 10 or 20 watts is simply overkill.