Another thought is that subwoofers and speakers are different sizes, so placing your speakers on top of your subwoofers might make them too high or too low for proper listening.
FWIW, I would never put my speakers on my Sub.😀
Speakers on top of subwoofer
Is mounting a speaker on top of a sub a bad idea? Specifically, a Harbeth 40.x mounted with isolators on each corner so the cabinet can flex/breath on top of a Rythmyk E15 I have a limited amount of floor space and the Rythmyk is a little taller than a Tonteger which is a good thing since the Tons are a little short to place the tweeter around my ear height.