Stacking subwoofers

You would be better served by adding just 1 IF you can place it optimally so as to eliminate any existing dips.  If you can't choose where to put it bass traps and EQ may be a better direction for you at this point.

Seems to me that one would be better served by spreading additional subs around your space, rather than making a stack. Adding more subs in the same spot does nothing to provide additional mitigation of any peaks and valleys in the rooms low frequency quality.

Sub stacking is a bit of a goofy phenomenon observed in some videos these days. Don’t get inspired by such videos and waste too much cash in that manner.

Depending on your room, setting up a single sub near-field (you may even be able to set your whisky glass on it) and some positional eq could nullify the need for additional subs (at times), i.e., when you are optimizing for a solitary listening spot.

You could also get discrete in-wall/ in-ceiling subs at the right locations. But, modifications to some simple dry wall and in-wall wiring can be extremely scary (terrifying) for some audiophiles.