Stacking

Yes, negative effect(s).  Perhaps too numerous and/or unknown to list here.  To include possible instability of a large objects.  Best of luck on a viable solution.

I hear people have been having success in putting their subs behind their listening position. Just a thought.💭 

Thank you for your post.

I think I'll have to buy a bigger home. LOL

I would never stack speakers on top of subs!! Way too many vibrations going both ways. If space is limited put the subs next to the speakers or stuffed in the corner of the room. Put them next to a couch or chair and stick a lamp and a plant or some coffee table books on the subs. Blocky end tables. 

@danmar123 Wouldn't smaller subs remedy the need for stacking?