And there's the question right there! Is it as seamless as it makes out because we've had lots of conflicting info on here.
Nobody has actually come forward with definitive proof that GLM will 100% align two subwoofers as one subwoofer then go on to full integrate them with your mains.
We've only had some text off Genelec (which has still yet to be proven) and lots of assumptions.
That's a whole bucket load of money to spend on a pair of subwoofers which may or may not actually work as well as we think they will.
Yes we've had a statement from Genelec but they don't actually state that the subwoofers in the subwoofer group will be time-aligned to achieve a flat response.
Too many unknowns for me to put my hand in my wallet really.
It doesn´t require more, everything else needed is already there: individual settings for individual subwoofers and individual main speakers, subs phased to same main speaker, yield optimal result automatically in most cases.
The result may leave room for improvement mostly then, if room is exceptionally bad and/or there is problems in phasing i.e. those multiple subwoofers are drastically different distances from listening position and/or from (that phasing) target main speaker. In those kind of bad/difficult cases no automation can achieve optimal results. Dirac & co are very propably much more baffled in those hard cases than GLM.
So, because very bad room could trick even GLM, it´s acoustical settings editor is always open for manual modifications if user thinks he knows better. As you must already know. We are talking WC/bathroom level -reverberation overhell -tier "acoustics" then, though.
In good or, at least "normal" room, GLM´s automation should handle it OK, no matter how many subwoofers are involved.
What comes to Genelec´s subwoofers SPL-performance, take into account that Genelec subwoofers are built to last
. They won´t let user to push them to high distortion, and eventually to thermal suicide. Like SVS, Arendal, etc typical consumer subwoofers. Which resemble a lot overclocking -PC-boys @ 100 °C -running computers. Definitely not sustainable in long run.
Consumer subwoofer vendors often let their products voice coils run on dangerously high peak power levels. Which, when pushed, leads to so called Final Snap, sooner or even sooner. This "let it burn, don´t give a shit" -attitude is opposite to Genelec´s sustainability, longetivity and absolute reliability -way of making their products.
Every time Genelec sub gives red protection indicator, consumer subwoofer without limiter would have let that power peak pass through to driver, to slowly torching its voice coil. Or to exceed its amplifiers safe thermal limits - the other typical factor home subwoofer vendors don´t give a damn. This may look bad in YouTube -quick review style of looking audio gear "subwoofer gave me a limiter, LOL", but experienced real audio people should understand the reason behind this.
So if you want the subwoofers, which last soundtracks nuclear explosions , mothership fly-overs or whatever you decide to feed them, for years to come, forget those Arendal/SVS etc. "overclocking" -crap, and rely on Genelec. Especially on subwoofers, they are under the most stress on loudspeaker system and as such most easily overloaded.