Designer's options e.g. Monitor 40.1 low frequency curves (generalised)
I found a picture on the internet which shows a generalised situation concerning the low frequency response of most speaker systems, sealed or vented. This is not at all specific to the Monitor 40 or M40.1 (all details are different) but it shows the general idea of the designer's options.
Note that there are a number of frequency response (sound pressure) curves that have been overlaid. Each one represents a choice for the designer who will eventually optimise the speaker system to behave as one and only one curve. Note that all curves pass through a point which I call the 'pivot point' at, in this generic example, 60Hz. You can see that if the designer wants some more output above the pivot point this can achieved done but the trade-off is that there will be less output below that point. In other words, he can trade extension for output around the pivot frequency.
Although the designer will fix the parameters of the bass unit unit and/or enclosure to whatever his marketing dept. tell him is the low end sound needed by the customer, if the box is vented (ported) the user does have a small ability to swing the changes around the pivot frequency. He can stuff the port(s) with socks or similar which will have the effect of reducing output above pivot frequency and increasing below ..... but he will reduce power handling and he will probably change the sonic quality in the middle frequencies as the box can no longer 'breath'.
The relevance of this discussion is how it relates to the M40 and M40.1. In general terms you could say that the M40 and M40.1 have approximately the same pivot frequency, and the M40.1 has been tipped in favour of extended bass, a lower Q, and hence more compatible with typical untreated domestic rooms. The original M40 was specifically designed to mimic the BBC LS5/8 and work in traditional BBC-style control rooms where the deep wall-lagging is tuned to suck-out lots of sound in the 50-100Hz region.
Hope that helps.
Alan A. Shaw
Harbeth Audio UK