I think you want isolation rather than coupling with a suspended floor, that’s been my experience anyway.
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You’ll just couple the bass more effectively to the floorboards!
Stu is right - you need to decouple.
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Chap who knew his stuff that I was on another forum with used to swear by these… Never heard them myself but far more sensibly priced that Gaia or Townshend options. Also handmade in Cornwall I believe.
Buy custom made magnetic levitation hifi feet (solidairaudio.com)
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I do still have multiple sets of Soundcare Superspikes, might give the a go although I think they are designed for solid floors?
You could go the Townshend-on-the-cheap route, and get some valve springs. Triumph Herald ones seem fairly cheap and look about right for your heavy speakers.
The idea behind a spring is that it couples at high frequency but not at low. So your speaker should happily wobble at 1-2Hz or so when on them. This should mean that bass is partially decoupled while above that is coupled.
The later Townshend devices used damped springs. I’m not convinced there’s that much difference, as with a resonant frequency so far below audible, inertial damping should be enough. Similar to a tonearm I guess.
Anyway, a reasonable experiment for £20 or less on eBay IMO
Springs decouple at higher frequencies otherwise your car suspension would give a very uncomfortable ride.The point of the Townshend devices was to stop vibration in the cabinet at audio frequencies further exciting a suspended floor.
Thanks guys, I’ll have a look at some options!
The sensible approach here is to lift floorboards under the speakers, excavat to a depth of >500mm inbetween the joists, pour in concrete and recover.
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You have it backwards. Below resonance of the spring/mass system it will couple, above that it will decouple.
It’s also impossible to generalise what springs should be used - the frequency is defined by the spring rate and mass. So heavy speakers need stiffer springs and vice-versa, obvs. But you need to know the rate to calculate the frequencies.
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Just move house again. It’ll be easier in the long run.
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It’s not like a turntable where the isolation serves to prevent structure borne vibration reaching the groove/stylus interface. You’re trying to prevent mechanical vibration in the speaker system exciting the structure that’s supporting it. In my experience, the Auralex foam pad approach is the easiest to implement and tune for the weight of the particular speaker cabinets.
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Get busy with the concrete, if you don’t do it now you’ll never be allowed to👌
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Ask @myrman what he did in his old house. He had a suspended floor.
He moved and built a new one.
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Concrete or bust
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Having a look online, I like the idea of these, any idea on stability with tall speakers on them on carpet?
I’d love to try them please 
Stu’s suggestion earlier is the cheapest and easiest way to decouple - it’s not technically perfect, but it’ll make a tangible difference straight away.
Squash balls are cheap, and I’d maybe use 6 per side as the Kefs look weighty, and they are best be sat in plastic castor-cups/carpet-protectors like this -
Get 'em from ebay or any hardware/DIY store.
You can just sit the speakers straight on the ball/cup combo, and that will work, or grab some paving slabs of a suitable size from the DIY store as well, spray them black (or whatever), and use those as a platform to add a bit of mass.
Then, once you’re bored and in a faffy mood, get another set of slabs, stick some sorbothane on top of the first set of slabs, new slabs on top of that - now you have constrained layer damping as well… With the extra weight a few more cups/balls are probably sensible…
At this point, the good lady informs you that it’s an eyesore and you go out and buy a commercial product…
Funz 
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I’d forgotten those existed (assuming they still do) but they work really well - think it was drum-kit isolators I used to use - Auralex “Platfoam” 
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They should be fine. I had LVs on them which while not the tallest were absolutely fine.
I took the spikes off the speakers and sat them straight on the Gramma mats.
They are in the original boxes, I will get them down from the loft this evening and see how to get them to you.
Either forum taxi or courier would work OK
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