Thanks, I said as much in post #130, another thing to bear in mind is the bass driver is crossing to a large MF/HF horn mouth roughly equal to the 15”.Fewer crossover points/ drivers mean fewer potential discontinuities and fewer steps away from a point source. Some leading monitor manufactures such as Geithian cluster the Mf/ hf in front of the bass cone( they don’t seem too concerned with a 550/800 hz crossover point range either), while others such as Westlake offer 5 ways (some with an 800hz crossover)on a baffle the size of Wales. FWIW a common rule of thumb mistake people make when estimating the wavelength/ cone diameter / max crossover freq is that for example Tannoy the 15” is actually 12.5”and the JBLs are approx 13”. Cone profile also has a bearing on the beaming ‘issue’. Loudspeakers present a myriad of issues and an equal number of compromises and design decisions/approaches often impacted upon by the transducers that are available to the designer. Porting is another chestnut where people claim they can hear group delay at 25hz or because they've heard bookshelf sized boxes ported to give fake bass all ports are bad.Honestly do you think ATC(for example) would invest so much R&D in improving their bass units then throw the benefits way in a 'sub standard' cabinet loading/tuning method?
The crossover frequencies are determined by the number of ways and by the intrinsic characteristics of the drivers.
With horns you need more ways due to their narrow operating range but the M2 uses a waveguide not a horn.
TAD floorstanders are 3-way and cross over at 250Hz and 2kHz,
The largest Vivids are 4-way and cross at 250Hz, 1000Hz, 4000Hz.
The LSR6332 crosses at 250Hz and 2.2kHz and needs to be partnered with a sub to achieve almost full-range (26Hz @ -10dB), crossed at 80Hz.
The ATC SCM150ASL Pro is a 3-way and crosses at 380Hz and 3.5kHz.
All use "assisted" low frequency extension (ports).
As far as I know, there are four major benefits for using bass reflex: cabinet size, load/efficiency, reduced excursion at tuning frequency, SPL.
The downside is transient response, although this can probably be mitigated to inaudibility by tuning the system to a frequency below 30Hz.
And as anywhere else in he chain the need for more accuracy in the sub-bass range depends on the listener's expectations as well as the type of music that he listens to.