Bookshelf Speakers Sitting On Dual Subs?

Stereophile:

While the 8c is big for a stand-mounted speaker, its cabinet isn’t big enough to develop a lot of bass; Hofmann’s Iron Law has not been repealed ... D&D takes advantage of placing the speakers close to the front wall with what they call Boundary Coupled Bass. When the 8c is positioned from 4" to 20" from the wall, the direct output of the rear drivers is acoustically coupled to the energy reflected by the wall such that they act as a single source ...

So it seems desirable to place D&D close to the front wall. Now the issue you have to cope with is whether which location is optimal for Elac sub. if you want to stack them up ...

I believe that D&Ds DSP correction will even take care of the sub’s behavior when the signal is run through the monitors (even though the 3070s have their own optional DSP).

This probably is an overly bold assumption.  If the sub is not in an optimal location and / or its performance is not optimized by its own DSP, relying on an external DSP in attempt to optimizing both main and sub simultaneously would possibly lead to a less desirable setting.  I thought a more logical stepwise approach would be to use the sub's DSP to optimze the sub performance first and then use the D&D DSP to optimize both.  But only you know the best.  This is a quite interesting, educational experiment and please update us on the results.