I went the route of having a two channel tube amp run the KHorns, and another tube amp (same amp, but wired in mono) run the center, which is a Belle..which I also believe does a better job as a center, since the specs match the KHorns. My tube preamp has two syereo outputs, so output A goes to the stereo amp, and output B goes to a mini stereo to mono box (homebuilt by forum member Tom Mobley) and then to the mono amp. The mini box has a built in attenuator so I can control the signal level to the center, which I have set to about 5db less than the KHorns. It's there, but it doesn't announce it's prescence. Like a properly set subwoofer.
I used to own a Scott preamp that had a built in mono output, which was less hassle, but I enjoy this set up more. There is a diagram on this forum how to build the minibox, if you're interested.