If you open the top, the drum should be on springs - you can shove it about to feel if something's assymetric (or fixed).
My machine is on a stiff tiled floor. Concrete floors are not immune either. If your floor is actually bouncing (resonating) then that's worst case of course. I called AEG and that's the first thing they asked: "Is it a solid or tiled floor", and then said to use pads. There's a really slight, barely detectable slope and it goes downhill, so I adjusted out the feet on the downhill side (the direction it walks). That more or less stopped it moving. I put a pile of cut carpet tiles under the feet which stopped it, but they're a bit of a nuisance so I took them out. It comes slightly forward so I just shove it back once in a while.
If I put something heavy in it like a rubber-backed mat, it gets excited and you can move it with a slight slide pressure.
The ideal solution - as with a car suspension, is to relatively increase the non-vibrating mass. In a WMac that'll be a block of concrete bolted to the case. But the more they make the drum hold, the less room there is for that, and the shorter they have to make the suspension springs. Bigger diameter drums exacerbate the vibration energy too of course. Top loaders are less heavy than front loaders, about 50kg rather than 70 ( I checked). That's against you.
If you're desperate, you need an engineered vibration absorbing material which is designed to turn the energy to heat, so rather than acting like a spring it's like a shock absorber. Car tyres are like that, you don't want a "bouncy" rubber. I have to use such stuff under high-mag microscopes. A couple of mine weigh 15-25kg (a lot) which helps, but you still have to isolate them from the building movements. A reverse of the WM situation, but direction doesn't matter. The best remedy is
Sorbothane(TM) from
Sorbothane.com . You look up the mass and the frequency to select for which foot thickness and material you need. You may find it rather expensive. You can get the mats on Amazon but they don't work the maths for you, use the maker's website. You'll need the higher Durometer stuff I expect - allow say 15kg per foot.
Your load per area of foot is quite high so really you'd use a spreader plate under the foot (like a carpet-protecting cup) - it gets more complicated.
A single sheet of Sorbothane would be most convenient, below those plates, but most of it would be wasted.
The distribution (rotation) speed of the machine is set to line the drum evenly with your basket of socks before it spins. In a front loader you can see the said socks just falling off the inside of the top of the drum as it goes round, so they roll around and spread evenly. It's drum size dependent, something around 30 rpm. (It's a standard school maths question.). The speed can be off if there's a fault, but you can't distribute a doormat.