From everything I've read, the KLF-20 and KLF-30 are much preferred to the KLF-10s. I finally found some KLF-20s on Craigslist about 8 months ago and picked them up in great condition for $400. I don't really notice any harshness at high levels, especially not in the upper ranges. The horn tweeter is accurate, clear, and not overpowering in the slightest like some people seem to think of Klipsch. I just did some high level listening last week, though I honestly couldn't tell you what SPL it was at. I find it harder to tell with my Klipsch speakers since they can handle quite a bit of power without distorting or sounding very fatiguing to me.
I used a Behringer EP2500 with signal coming from my Yamaha DSP-A1, and the Behringer certainly has no shortage of power, especially considering the 100db/w efficiency of the speakers. Though I think I have read that some suggest damping the mid-range horn on the 20s and 30s, the KLF-10 might have the same horn.
Here's a video I took last week; the microphone distorted pretty badly at these levels but I noticed not audible distortion from the speakers themselves. I was hesitant to throw more power at them only because I don't have a subwoofer connected at the moment and I'm not sure how much excursion the woofers are capable of.
https://www.youtube.com/watch?v=qkXcv7hjsb4
