I haven't been on this forum for quite some time – didn't even realize that Spatial had established a dedicated forum here!
In any event, I wanted to respond to this topic. I have always used my Spatial speakers (first the M4, and now the M3 Turbo S) with a low powered, SET amp. In my case, it is the Audio Note Kits "Kit 1" Amplifier, which is 300 B-based, putting out around 8 W per channel.
I have always found my amp and the Spatial speakers to be a wonderful match. The sound is sweet, open and lifelike.
That said, I am currently working on a comprehensive review of my M3 Turbo S, for my audio blog. For the first time, I'm pleased that I'm going to be able to offer a comparison of how some very different amplifiers drive these speakers. Specifically, I managed to borrow a high-powered Krell solid-state amp. Once the review goes up in a couple of weeks, I'll have some more data by which to compare a low powered SET amp with a very different solid-state amp.