Steve, there are a lot of amps available. I have C7's and (discontinued) K6's. I use, among others, a Muse 160, 160 wpc, high current. Havent seen one for sale in a long time. Reviews and retail dealers said it was a little soft on top, which is what you want. What you dont want is an analytical or bright sounding amp, so whatever people recommend here, pay attention to how they describe the sound or look for reviews. Some of us like purely neutral amps, but nobody with these speakers likes any excess on top.
I saw recently a Marsh A-400 amp on sale here, and some Conrad Johnson preamps. Also some Anthem preamps. Classe amps, if you see one here, are good. There are some I would stay away from, but not wanting to offend any fans, I won't mention them. Feel free to use the A'gon emailer.
You might also try the official Harbeth users group, which you can join through the Harbeth.com website.