Smatsui,
I appreciate your experience with both SHL5 and M40.1 in the same room and with the same equipment. And I agree about the significance of WAF that varies with individuals.
Yes, my room has a ceiling height of 8 to 9 feet. Your room certainly has a very high ceiling, and I can imagine the M40.1 to load the room more effectively than the SHL5 with the added space. I find placement of larger Harbeth speakers to be critical for an even bass response and 3-dimensional soundstaging. They need to have some space from wall boundaries to sound their best or some uneven boomy bass or "flatness" may be perceived. The bass of the M40.1s in your room may be "better" than the SHL5s, but in smaller rooms with a common ceiling height, the bass of the M40.1 may overwhelm the room. Placement of the M40.1 is far critical than the SHL5s in a mid-sized room, from my limited experience with my friend's setup. The placement of the M40.1 has been changed numerous times with the speakers pushed out from the walls, back nearer to the wall, and distance between speakers fiddled many times just to get an even bass response. For a fussy listener, the setup of the M40.1 ain't as easy as it appear to be.
And I believe room treatments, especially bass traps do help improve the bass quality a bit especially in smaller rooms.
I am not denying that the M40.1 is an excellent speaker and may very well be superior than the SHL5. My point is they may be a fussier speaker to set up than the SHL5 in smaller to mid-sized rooms, and can disappoint in some setups if compared to the smaller Harbeth models. As usual, large speakers produce more bass, and more bass usually equates to more problems if not properly addressed (unless the listener is not too critical on this aspect).
Cheers.