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.
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.