you will be fine with the CS 600. I've used as little as 32 WPC (tube) with both the SHL5's and 40.1's in a room much larger than yours. Power IME is over rated.
Harbeth 30.1's w/Leben CS-1000P or CS-600
I have Harbeth 30.1's in a small 10'x13' room and am currently running them with solid state Blue Circle gear. I'm curious about Leben but fear even the CS-600 will lack the power needed to properly drive 30.1's. Does the CS-1000P buy you better sound or just more power? The cost of the 1000P plus their preamp would be nearly triple the 600, but if it's a huge upgrade I could potentially swing it. Hopefully I'll take a trip to NYC and hear both in the near future. Any thoughts? Thanks.