If you considering SimAudio Moon for the Harbeth check out the Benchmark AHB2 amp. The SimAudio is likely a very good choice and something I would take over a warmer toned amp. I heard a Harbeth speaker sometime back with Luxman gear, maybe 509x or m900u and c900u. I liked it a lot but thought it was too warm for my tastes and it would get tiresome. At that audition, I was thinking a Benchmark AHB2 would be what I want to hear with a Harbeth.
I have a speaker that looks a lot like the 40.3 and it is slightly warm. I am going to have a listen to the new SimAudio 861 amp with these speakers.