avantgarde duo spikes or plastic feet ?

Both feet and spikes will transfer energy to the horns. Try keeping all the bolts loose. This will tranfer less energy. You may get slightly better bass with the spikes

The spikes (if they're any good) will transfer the resonance energy into the flooring system which is a very good thing. The plastic feet, on the other hand, are of no use.

Ensure all bolts taught or tighter, otherwise the vibrations will become trapped as a result of inferior or loose mechanical connections and since those vibrations (captured in a moment in time) would have nowhere to go, they can only dissipate over an extended period of time and exponentiate because of the poor connections. Sonically, this is not a good thing.

-IMO

Some guys use 3 Black Diamond Racing cones under the subs, lifting them off their feet.

Try the Grand Prix Audio Apex footers. Pricey but they work marvelous with my Duo Omegas. See 6 moons review.
Good luck. Contact Walter at fidelisav.com if interested.

Hands down the GPA Apex footers.

On the rails, not the subs.

Horn cylinder and sub mounting bolts tight.