An 'old' thread but .... there are some observations still relevant today.
I have an older version of the Spendors---the SP-100---and I have never heard the Harbeth 40.1, so my comments are not exactly what you're seeking. However, I wanted to say that Spendors have a reputation for being more compatible with tube amps than do Harbeths. This seems to be true across the model lines but particularly so with the big models.
The Spendors have higher measured sensitivity which certainly helps, but they also have a less demanding impedance than Harbeths. In my case, I have used the SP-100s with a single ended 300B amp, 8 watts or so, as well as higher powered push-pull amps ranging up to 50 watts. The Spendors seem quite comfortable with all of them. My current SET is only 1.5 watts and I will admit it is a bit underpowered, but even so it does a surprisingly good job with most music on the big Spendors.