FYI, you can isolate any speaker cab with a cheap amp stand. Just getting it a few inches off the ground disconnects that bass like a champ.
You haven't isolated the cab, of course. All you've done is reduce (not eliminate) half space bass reinforcement. And while that's a good thing, it doesn't solve the whole problem.
This is because you still haven't truly decoupled the cab from the floor, since the stand's legs will vibrate and transmit the vibration to the floor to some degree (of course, this depends on the design of the stand, but most stands do not have decoupling material that actually works).
And so you'll still have structure-borne vibration that can show up as low frequency mud, or cause structural buzzes and rattles. Concrete floors will vibrate just as readily as a plywood or solid wood floor, and in fact, because the concrete is poured in one piece, the structural vibrations can be even worse, where a wood floor vibrates in pieces-parts, and there is some dissipation of the vibration as it moves around the floor.
This is just the same as when a mic stand will vibrate from the floor, via cab vibrations and footfalls - which is why, for example, serious studios always use shock mounts on good microphones. Heck, I even use shock mounts with an SM57! The reason isn't to protect the mic - it's to get rid of the smear and mud.
To decouple the cab from the floor, the Gramma foam stuff works pretty well. So if you want true decoupling, and the elimination of structure-borne low frequency vibration, which does cause noise, the Auralex Gramma is a good product. It's not perfect, but it's not bad, either. I haven't yet found a perfect decoupling product. At one time I had ASC make me a subwoofer stand with bass trapping material inside, for the cabs. And it worked well, but not significantly better than the far less clumsy Auralex stuff.
Often when I record guitar, I use both a Gramma to isolate the structure from the cabinet's vibrations, AND I use a riser to lift the cab off the floor.
AND I surround the cab with gobo bass traps that both isolate the cab and absorb excess bass.
And still, it ain't perfect. For perfect, you need an anechoic chamber with the cab suspended about 3-4 feet above the floor, from hooks that are decoupled from the ceiling beams and...you get how difficult this whole thing can be...
One studio I consult with has its recording booth floor completely floated on a special acoustic rubber isolation platform - and this is a big room about 20x25 feet. Over the rubber there's a marble floor. Only rubber touches the edges of the control room.
The control room's monitor supports are concrete blocks sunk ten feet into the ground and they, in turn, are completely isolated from the rest of the studio's floor by acoustical rubber surrounds. This studio was designed by the famous Russ Berger, and the construction costs alone were over a million bucks for these control/recording booth rooms back in the 1990s. The place has three sets of these studio rooms! I can't even imagine what this would cost today.
Best sounding rooms I've ever recorded in.
