Bookshelf Speakers Sitting On Dual Subs?

NO, place the subs where the bass response is best (see earl geddes guidelines on my systems page)

Place the speakers at a 28 to 33 degree angle from the MLP. Do NOT place subs in the same location as your speakers to get best bass response, you are defeating the purpose.

True dat. Sub placement is optimized by the room, not aesthetics.

Vibration is a killer of high sound quality. Subwoofers are vibration devices. So if you want good sound quality, no. You want very sturdy, heavy (typically filled with steel shot) speaker stands to place the tweeters at the exact listening level. Isolation devices improve sound from already solid footing. As also pointed out you want to optimize both speaker placement and subwoofer placement and there is nearly no chance those will be in the same location.

Placement.  It's super convenient to be able to place the subs for minimum room mode interactions, and the satellites for best imaging.