Klipschorns have a sensitivity of about 101 dB SPL / 2.83 V / 1 m. Anything that scores around 105 dB SNR ref: 5 W / 4 ohm would be a good candidate (that's about 25 µV worth of noise across the audio bandwidth), and 100 dB should still be quite acceptable depending on listening distance etc..
A line-level attenuator between preamp and power amp is quite likely to be required. As much as 20 dB may be advisable if we're talking a typical 26-29 dB power amp gain and the preamp isn't the lowest-noise thing in the world (but might have plenty of maximum output in return), though 12 dB would be a more typical value. Only a few power amps ship with particularly low gain settings out of the box, the Benchmark AHB2 comes to mind (which also is why it manages to lead the 5 W table). OK, that one may be a tad over budget.