Dampening / stabilising washing machine on 2nd floor

[optimystery]

My previous Bosch washing machine gave up after about 12 years, so I've bought a new one.
This one seems to shake / wobble / vibrate a lot more.
It's not 'walking' or banging, but I imagine it can't be good for either the washing machine or the floor.
I have removed the transit bolts and plastic washers that it arrived with.

The washing machine is situated along an outside wall under a counter top, and this is a 1900s second-floor flat.
The floor vibrates a ridiculous amount.

I have it on 9mm plywood, levelled with compressed cardboard in the corners to get it level before sliding the washing machine in.
I've also added the neoprene feet cups that I was using with the previous machine (which worked pretty well).
However either with or without the feet the machine noticeably wobbles from side to side when tumbling, and shakes violently when on spin.
These newer machines don't have a spin speed dial, only programmes, so I've set it to Easy Care (1200 rather than default 1400rpm, which is extreme).

I'm wondering if a granite slab (2.5cm thickness is probably the max I can get away with) would help here?
Would something thinner like a large porcelain tile be liable to crack?
My reasoning is that even if vibration transfers through the slab to the joists, the slab would at least be dimensionally stable, and so the 4 feet would be transferring a balanced force downward, rather than individually.

Bosch recommend on their website to screw a thick piece of ply (min 30mm) into the floor, but I'm not sure that would fit under the counter, and seems like it would flex more. On the other hand, maybe having it attached directly to the joists might improve stability?
Other more extreme options might be to brace / sister the joists, but that's a bigger job without a guarantee of it working.

I've done a lot of googling and researching, and haven't found any definitive solution. Since I don't have the option of moving the washing machine, either fixing the floor or getting a stable platform seem like the only options.

Any advice from experts on here? How would you approach this?

 

[cross thread]

I purchased a inflatable bag that fits between the work top and washer ,definitely helped ,can’t remember the name but sure you could look it up.

 

[foxhole]

Cardboard is totally unsuitable under a washing machine.18mm ply would suffice , screwed and glued to existing.though if the machine is not levelled with a spirit level in both direction it will be unstable regardless of flooring .

 

[optimystery]

How would you approach getting the floor level if screwing/glueing 18mm ply to a floor that may not be level?
There is a thin layer of chipboard under which floorboards. These may also not be level!
It's much easier to get the floor level and just slide the machine in, because once under a counter top I cannot access the rear feet or get a spirit level in

 

[optimystery]

Just to clarify, the cardboard pieces are small folded (totally compressed) 'shims' under each of the feet (below the solid ply) to get the board level.