Subwoofer on Top???

Subs put out low frequency bass. Duh. Low bass bounces around the room creating areas of reinforcement and cancellation. You can walk around a room and easily hear places the bass is too loud, or too quiet. Or instead of you moving around you can move the sub around which will move the modes around. This works in 3D. Its just most people aren't into lifting hundred pound subs. Anyway, that's all that's going on. He could get the same results putting the sub somewhere else, it just takes a lot of trial and error to find that place. In other words there's nothing special about the sub being close to or on top of the floorstander. In fact, the best solution for really good bass is a distributed bass array. Four subs placed asymmetrically around the room. Try that and you will never go back.

The second part, the "sub" thing is, It's probably doing more than just SUB bass duty, Above 60-100hz things start getting pretty directional..

Third, you really don't want all the vibrating and such ON the monitor cabinet.. Tweeters and mids, really don't like it, The reason I separate the two. 300 hz and down are separate cabinets, actually two separate cabinets 15-100 hz and down and 100-300 in a band pass, different enclosures. Very little vibration on the monitor section because of it..

Regards

I run a Bag End 18" subwoofer on top of the 2-10" amplified Bass rig but this is more of a stage issue that lead to multi-sub home experimentation. 

Crawl testing for sub location has never been near a main speaker location in my rooms but one is elevated 25". Give it a try.

I'm all for raising it, as a matter of fact I encourage that. Just not on top of a monitor cabinet... The other way around, maybe, BUT I'm still in favor of separate boxes if you can. It eliminates the back side of the driver being influenced by other drivers in the cabinet, even though it is in a separate chamber. You only have to deal with the front, and phase plugs will do the that for bass drivers.
A VERY narrow tapered baffles will take care of,  Mids and highs

Regards

It's not the sub location, it's the additional mass on the floor standers.

Best,

E