I'm sorry I missed this question.
The reason is simple. It is because the design demanded it.
These speakers contain not one but two TLs.
The bottom two drivers are in the long TL. Both drivers handle the below 60 Hz signal, including the LFE channel. The upper 10" driver also handles the BSC for the MTM drivers.
The line containing the MTM drivers is tuned an octave and a half above the longer line containing the 10" drivers. The port of this line is above the upper 10" driver. The ports for the long line are either side right at the top.
This is the smaller line.
The longer line was built round the shorter line.
The MTM produces excellent focus and imaging and limits ceiling and floor reflections. It also allows two drivers and therefore motor systems to handle the crucial 80 Hz to 2.5 KHz range, were the vast amount of the power is. It is NOT below 80 Hz. Subs offload speakers NOT amps and receivers. So this really helps to limit thermal compression. The crossover between the two lines is entirely acoustic. The low pass 60 Hz is fourth order. The BSC upper crossover is first order transitioning to second, with the electronic crossover have the inverse response to the driver response. There are no passive components in these crossovers. The mid tweeter crossover is passive.
This is the response on axis.
You can see response is extended to 20 Hz and being a TL rolls off at only 12 db per octave below that point. Two 10" cones have the same radiating area as one 15" driver. However much of the bass sound is radiated from the pipe ports. You can see the acoustic crossover of the lines between 35 Hz and 60 Hz. The rise at 4K Hz I'm not certain is real. There is no hint of sibilance or forward balance, so I have left well alone. It is very hard to measure a long line of drivers like that. The bass output is prodigious and tight, and no dedicated sub is required.
The reproduction is non resonant and the phase response excellent as can be seen from the impulse response.
If there is another speaker out there with an impulse response as good as that, then I'm yet to see it, or be made aware if it.
The listening test confirm it is so, as the speakers are tight and true to life with no audible ring or overhang.
So the driver layout is totally determined by the demands of the architecture of the pipes.
This is the center TL line.
It uses two coaxial drivers, with the lower one which is the main driver is right above the screen, the second driver provides BSC via an active network. Originally I did not use the tweeter in the driver, but subsequently used it to pull up the response above 9 KHz, and also take out a slight dip at 9 KLHz.
The surrounds are 2.5 way sealed units.
The rears are also dual TLs. You can see the two ports, on above the bass drivers, the other at the top of the speaker.
All powered by this.
