Even more than with most audio topics, there's a real lack of consensus over what kind/power of amp pairs best with Harbeth, particularly SLH5 and 40.2. Take a look over, for instance, at the Harbeth User Group website, or other threads here.
Several people point out that you often see H shown with Luxman. Vinnie Rossi's amps also produced wonderful results.
The only generalization that seems to hold is that a bunch of people report good results from a really widely differing range of amps.