I am currently talking to a customer about this exact speaker with a desire to driver it with my Diavolo which is a 10W SET. Incidentally I am advising against that.
The issue is headroom for dynamics. The question is at what level do you usually listen and how much headroom do you need. My rule of thumb is that most people list at about 80dB for reference listening and you want 10-15dB of headroom for music. For home theater reference is 85dB and 20dB of headroom is essential. 10w will deliver 92.3dB of volume and most of the time, headroom will be adequate but every now and then musical dynamics are going to push the amp too far.
For those speakers in a small to medium room, 18w will give you 94.9dB of volume 10’ from the listening location. For most music this is plenty assuming you are not turning up to 11. I chose 18w because that is the power output of my Carissa SET. 32w will deliver 97dB. Fine for music.
Alternatively, for home theater where you really need to be able to hit 105dB, you need 185w of power. That means a high power SS most likely or a high powered tube though I tend to prefer high power SS amps to high power tube amps. Musical Fidelity is an amazing match for Harbeth.
It comes down to how you are going to use it.
