I don't have an ACI sub but do have subwoofers and play with spikes often. Their use, and the success of their use, depends a lot on what type of floor you have & what material the spikes are made from.
Any which way, you will lay the sub on the floor...the spikes will be inbetween the sub's enclosure and the floor though.
One simple rule of thumb is that if you have hard surface flooring then soft rubber feet would be best. Or using a pad like the Auralex Subdude ($50). For sub users on hardwood floors, these pads can make or break sub enjoyment. Coupling a sub to a bouncy old hardwood floor is not optimal.
For those of us living on carpet with a solid subfloor, then spiking can be advantageous. Personally, my stereo pair of powered subs came with rubber and metal spikes. I use the rubber ones sitting right on my carpet...no connection to the concrete subfloor below the carpet.
If you have concrete, do not spike to it. it imparts horrid sonics to the sound... same goes for your rack too.
If you have hardwood floors, consider a SubDude pad from Auralex or soft rubber feet for the sub.
Typically factory spikes are not meant to drive vibrations from the sub into the floor, they are meant to hold the enclosure steady. As time goes on hobbyists are realizing that most factory spikes are inadequate. This shouldn't say anything about the mfgr though, they have a price point to meet. Its the same as why amp builders don't send fancy power cords with their gear.
Luckily, the market has responded and there's a host of accessories to plow into now.
If you're on a hard floor surface and need absorbant feet, look to some household items like neoprene, to initially get you experimenting without spending much or any $. then, when you know what style of footing (coupling or de-coupling) will work best you can narrow the field of choices.
with subs especially, its all about finding out what works best for you.
