Stacking speakers with the same impedance characteristic maybe a cost effective way to enjoy this hobby.
I didn't mention that the first picture shows the large Advent and the Advent/1 (smaller Advent) are stacked in parallel.
The KLH 23 with the Advent/1 on top are not stacked due to what I think is the different impedance of the components, I have not tried, however.
Ideally, back in the day, people liked to stack two pairs of large Advent, the Absolute Sound had an article on this. Since I have no luck or space for another pair of Large Advent, I settle with the current configuration, and it works amazingly well.
Evidence showed folks also like to stack Bozak 302 to get the Bozak Concert Grand effect, but that would be too labor intensive for me.