Folks in this thread dismissing "subwoofer stand" positioning, despite lots of well-regarded commercial examples and experiential evidence to the contrary. If this was such a flawed topology, why did we ever have tower speakers in the first place? By this logic, then every pair of tower speakers capable of putting out deep bass is fatally flawed.
I'm not familiar with those Genelecs. At first glance it seems like the dual drivers achieve some sort of dipole thing and I think the intent is that you wouldn't need more than one unit. I'm not sure if a second unit would be beneficial, detrimental, or merely superfluous.
As for crossover slopes, a steeper slope may cause audible discontinuity particularly off-axis as the subwoofer will have a different dispersion pattern than the speaker to which you're crossing it over. On the other hand a shallower crossover slope probably opens you up to some more phase/cancellation issues as in my "80hz @ 12dB/octave" example where the subwoofer's playing somewhat audible content all the way up at 160hz. Bit of a cop-out answer but ultimately it comes down to experimentation with one's room and equipment I guess.
The one thing I can say without reservation is that lower crossover points are always beneficial assuming the main speakers are capable enough.
There are technical reasons for why the separate bass-system solution is superior to the old-school full-range speaker. I would not necessarily call it flawed, because it works - sort of, it is just that alternative solutions are so much better.
Here is the fr for a traditional speaker, and the fr for the new system that replaced it:
In the old system, the bass is made up of a couple of strong reasonances, very uneven response. No eq can fix the 60-80hz dip. Of course, all full-range speakers in a room are not like this, but something quite similar is common - uneven, colored, resonances. The new system is essentially flat.
When we look at what happens in time, it gets really interesting:
We see that it is not only the steady-state frequency response that was compromised, there was severe problems with group delay and timing as well. (This spectrogram plot shows frequency along horisontal axis and time upwards, sound level is visualized with color. We see what happens to the sound in time across the frequency range. Ideal is a thin, straight horizontal line at t=0.)
Fixing this with eq will be difficult, using just minimum phase eq will not work. Then the idea of acoustic treatment pops up, only to be dismissed when you realize anything that actually works will require rebuilding the room.
Fortunately, there is a realistic and very good solution. The bass-system here consist of 2 small subwoofers located where they could fit in, and according to advise given. Not necessary to try every possible location and measure. Since this bass-system has dsp with proper eq capabilty, it was easy to simply remove the only resonance that was left. The result is a system with smooth frequency response and good timing with no resonances across the whole bass range. Solved.