Bookshelf Speakers Sitting On Dual Subs?

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

Don't the 8C's include subs (dual 8" drivers) on the backs of the cabinets that cover 20Hz-100Hz.

Is there another model without the built in subs?

DeKay

8cs do have the rear 8" drivers which are great, but this is for a bigger room and to fill in lower end.  The 8cs apparently do rock, but this is a large and oddly shaped room.  Note to above posters pointing out sub placement issues:  DSP, DSP, DSP.  I am well versed in manual sub placement by ear.  DSP is not perfect, but it's pretty damn good these days for sub-optimal room situations.  As for vibration issues: essentially zero - the 3070s are rock solid.  With isoacoustics stands on top with the 8cs on top of those, I suspect zero vibration from subs will get to the 8cs. 

This would be the ultimate test for the isolators you plan to use.  If it works call them and offer to make a commercial for them.

I had some Axiom M22TI bookshelf speakers I used in this manner and didn’t feel it was a negative. The dual subs help cancel standing waves, and as long as the tweeter height is satisfactory let your ears be the judge. I had used some Audioengine stands for my setup as they provided some tilt. I am running dual subs for my current setup but they live behind the primary speakers now, not under.