I have always heard you isolate with concrete and couple with wood floors. Might be an urban myth. Especially since you notice the exact opposite effect. Wayne should reproduce the original post about mass loading suspended floors; it's on here somewhere. I just remmember seeing pics of Gizmo with his Tannoy Westminsters bungee-corded to twenty cinder blocks as stands. He called it mass-loading. I don't know but my Valencias roll off pretty high up there; yet I sure hear the sound of a string bass low E note well.
Shouldn't the low A on a piano be the target, rather than the low E on a bass guitar? The A on a piano is 27.5Hz, I believe. Or, maybe its ok because the second harmonic is at 55Hz which most speakers can handle.