Thiel CS 2.4 Vs. Harbeth M40?

I have owned the 2.4 and now use the Harbeth Super HL5. I have not heard the Monitor 40 but know that it shares the same Harbeth family sound while delivering a lot more bottom end and having a better tweeter. My understanding is that the Monitor 40 can produce too much bass for small or medium-size rooms.

You are talking about two very different and I think equally valid, exceptional approaches to loudspeakers, although the Monitor 40 is almost twice the price of the 2.4 (a different Thiel model may be a better comparison, or compare the 2.4's with the Harbeths I have). The Thiels will give you greater detail; more precise imaging and greater resolution; a tighter, more tuneful bass (the Monitor 40 may go lower and have more bass impact, however). They will also be more revealing of all your upstream components. The Harbeths will give you extraordinary vocal reproduction and instrumental tone. Music sounds more "beautiful" through Harbeths. Is this additive? Hard to say. Are the Thiels more accurate? Maybe. Probably. Which is the better choice? You need to hear these speakers for yourself and see which delivers the musical goods for you.

I'm glad to see someone else use the word "tone" to describe what the Harbeths (and Spendors) do well. The ultimate question is, "Of these two speakers, which puts a smile on my face with the music I love?" When I first bought my Spendors, I went through the whole, "But are they as accurate as X brands?" thing. But I soon realized that they made me listen to music for hours on end, something the "accurate" speakers I'd owned never could do. I would also look at the smaller Harbeth brands (based on room size and amp power) as well as the Spendor SP2/3, 1/2, and 100.

To be fair you didn't indicate what amp you'll be using. For the Thiels it is essential to drive them properly i.e. with an amp that provides sufficient current.