Anyone listen to Zu Audio's Definition Mk3?

No worries about either one. Even if the spike gets between the grates on the sub(coupling disc falls off), the spike is sufficiently wide that it is stopped on the aluminum plinth before it gets close to the driver.
Regarding stability, I have given them a good shove and they are very stable. It would take quite the push to knock them over. 150 lbs on a tripod is a stable platform.

I'm aiming to put one Symposium Acoustics Rollerblock Jr module at each corner of the two spkrs, and place them on Svelte Shelves. My Symposium Isis rack has been fantastic at isolating my main components, and I'm hoping I'll get a similar level of performance with my Zu Def 4 spkrs.

Thanks David...your explanation was helpful. I can't seem to find any images of the bottom of the Def 4's, but what I can see of the sub/plinth from the side it looks like there would be plenty of space in the corners for the 2 spike end, but that the single spike side would necessarily not be in a corner and have considerably less area...think circle circumscribing the inside of a square; the corners of the square have spare area, whereas the midpoint of the side would be touching the circle (very close anyway). Is this correct?

German,
Go on the Zu website and click on the Def 4 "Gallery". Scroll through and you will see images of the bottom.

Markpao - thanks. It does confirm what I expected...not alot of room for the 3rd spike. Oh wait...there's an aluminum "grating" on the bottom. Is that correct? That would alleviate a fair amount of concern.

Thanks.