Hi,

Yes what your neighbors are experiencing are solid noises, in opposition to aerial noises. Like footsteps versus conversations.
To schematize the lasts depends (roughly) on the thickness of the walls, the first on the density of the material used to build the structure.

You need to isolate the feet form the ground with the most possible absorbing material: You can cut little dots in washing machine anti slip rubber mat.
But it may not be enough. Complete fix would not be pretty tough: it will consist of tennis balls, cut in half where you would add a little top cover made of the cover of a Nutella pot or similar for example, and that you will place under each foot. Or the half ball reversed, with the round face upwards and nailed or fixed by some mean to the leg of the A9.

The rubber nature of the tennis ball will absorb most of the shocks (well know technique from electronic drummers).

They are mini tennis balls, half the size of real ones which can be less distracting. Of course full size balls would absorb more and you may find them in black. 

In any case try to push the sound a lot while around to be sure the A9 will not dance to much on those new shoes!

You can probably find on Amazon noise isolation feet that you can stick below the A9 feet to minimize sound traveling down the wooden feet from the A9 onto the floor and then traveling to the walls etc to your neighbor. Only you can decide if the effect on the aesthetics is worth it. You can also make sure the A9 is not is a corner where the port is hitting the wall that goes across to a neighbor. The A9 has a lot of power and moves quite a bit of air in the process - especially the back port. 

Here are the canonical products available on amazon and elsewhere, from purpose-designed to generic: (1) SVS SoundPath Subwoofer Isolation System (ASIN: B00NCSQ5GK) and you drill holes into the tips of the legs to screw them in.  (2) One of the various styles of "IsoBlock Silicone Isolation Feet" (incl. ASIN: B07QPBNKCL).  You could use the largest style ("Bigfoot") upside-down but carve divots into the tops (i.e. bottoms) to hold the legs so they don't tip over/out.  Same idea as @matador43's tennis balls above, and maybe harder to stick but more domestic looking; you might have to drill double-ended screws into the legs to ensure they hold as spikes.  (3) Speaking of spikes (e.g. ASIN: B07B4B68TC), which I don't think are worth much but they mount with tape and look OK; the problem is making them work at the legs' angle to the floor.  (4) Washing machine feet of various prices and quality: a fancy sorbothane set (ASIN: B00536VQE0) is pricey for what you get, but at least they appear to have thought about the material & design.

All are hideous, even the tiny spikes.  Consider instead an "art" rug of modern manufacture, so it's broadloom, extra-thick&heavy with multiple materials giving density changes to help shift the vibration -- as opposed to a gossamer, flexible genuine hand-knotted oriental rug, which would look beautiful but not work as well.  You can even hide an additional carpet pad under it.

(In a perfect world, there would be a double-ended bolt&nut cylinder made of rubber that you screwed directly into the A9, **so you couldn't see it** and then screwed the leg into that.  They make such dampers, to hang HVAC systems, but they are usually engineered for heavier weights.  As a result existing goods are too-large diameter and have just a single plate buried inside the rubber cylinder, made for nuts on threaded rods inside and hanger-plates outside.  STB Brackets take note: requires custom manufacturing, but easy to engineer...)

Remember, these suggestions only work to reduce structure-borne noise.  As @Jaffrey2230 points out, the bass itself can induce noise in balloon wall construction, and is very hard to fix unless you own the apartment and can do renovations.  Remember the last time a "rice rocket" with subwoofers vibrated down the street near you?  Same principle for your neighbors, if you turn the volume up and have anything but a concrete firewall between you and them.  But the inverse-square-law works (although frequency does matter, see: https://physics.stackexchange.com/questions/194526/does-the-inverse-square-law-apply-for-all-frequencies-of-sound) so maybe just a few inches further away will do the trick.  If you are still friendly with your neighbors, go next door and listen, while asking someone to move your A9 slowly until peace & (relative) quiet reigns.

Sorry to be so long-winded, but hope this helps.  Take a picture of what you finally end up with and post it here!

Thank you everyone for your thorough answers, I really appreciate them. I will read them in detail and start testing what will work the best. I will post my solution in this thread when I have solved my issue.

Thanks again 😊