Concrete weakens and enormously muddies the bass while rolling off and harshening the treble
I suppose "rolling off" and "harshening" could be reconciled in terms of, say, an emphasis in the 3 to 6kHz region followed by a rolloff above that. But I can't imagine how the surface that is immediately under the speaker could produce any kind of effect like that.
It has struck me in the past that more than a few of the statements in their catalog are, um, unlikely to have universal applicability.
I do, though, use their four-inch maple amp stand and isoblocks, and I've been pleased with them.
Regards,
-- Al