A good sub has a solidly built cabinet that has a relatively inert enclosure, however, to prevent scuffing the subwoofer top, and also to prevent movement of the speaker from rocking motion, I would use some rubber shelf liner like
this stuff. Cut off a chunk and place the speakers on it.
But an advantage of using multiple subs, especially when used as speaker strands is that you can use them at higher frequencies than 80 Hz without localization penalties. Most systems use a 80 Hz crossover frequency from sub to speaker because above 80 Hz, the sub draws audible attention to its position, and throw the soundstage in one direction. It doesn't sound good. Below 80 Hz, it is hard to tell where sound is coming from. But that isn't a problem with subs as speaker stands, you can use much higher crossover frequencies and the sound can remain natural and balanced. What is nice about that is you can take advantage of the subwoofer's much higher dynamic range in that frequency band. From 80 Hz to 100, 120, or 160 Hz, it can punch way harder than your speakers, whether they are bookshelf or tower. It also takes a load off your AVR amplifier. The sub's amplifier can handle that extra range just fine, a lot easier than your AVR amp.