I’ve owned the Harbeth M40.1 speakers for the past 12 years and I recommend driving them with a powerful solid state amp with a tube preamp. I’m using a McIntosh MC452 (450 w/Ch) with an Aric Audio Motherlode XL balanced tube preamp. In my opinion, the REX 3’s 80w/Ch isn’t enough for large scale music at high volume with the relatively inefficient M40s. By combining a powerful SS amp with a high quality tube preamp you get the best of both worlds. My preamp uses 6SN7 tubes in the preamp stage and 5U4GT rectifier tubes that give me a lot of options to tube roll.
Here is a discussion of the M40.2. https://www.whatsbestforum.com/threads/harbeth-monitor-40-2.25222/
The author uses 2 Benchmark ABH2 in bridged mode that puts out 400w/ch.
Here’s another discussion about the power handling of the M40, unfortunately the video is no longer available.
The CH Precision power meters in the video were showing an output of 700+ w/Ch. Alan Shaw was shocked by how much power was going to the M40s. I have a large room but I don’t play my music nearly as loud as the video showed when I watched it several years ago. The power meters on my MC452 have shown that the M40.1 speakers draw a lot of power. I think you could get by with an 80 w/Ch amp in a small room and not too loud volume levels. Even so, I would get 2-3 times the power that I think I would need to provide a lot of head room so that the amp is at ease at loud volume levels. You can see my living room system in the virtual systems. My 2 cents.