There was an article on this problem recently in HIFICRITIC and it was solved by isolating the speakers from the floor by an air suspension system. It is not sold over here and was somewhat expensive but you should be able to make up something that would work fairly cheaply using rubber innertubes. I would use the slabs Elisabeth recommended and use a tube of good size between them; do not over inflate them. This system also improved the bass in the system they reviewed and greatly lessened the sound transmitted through the floor.
I NEED a tone control to control bass.
The problem is, I'm living in an NYC apartment with suspended hardwoods and a downstairs neighbor with very sensitive ears. I have no interest in rolling off treble, but bass is another matter, as it varies from recording to recording, track to track, and gives me peace of mind to be able to adjust it on the fly, especially late at night. I know tone controls are taboo in this day and age, but wondering if any one has any suggestions on a way to introduce a high-quality bass control, probably subtractive, without degrading the signal path too much and without breaking the bank? Anyone know of a possible custom builder for this? Would also consider a bass-and-treble control if there's a good alternative out there these days...