Another approach which may work for you is to strengthen the floor from beneath. If there is a basement or crawl space, it is not difficult to place a beam across the middle of the floor, one under the speakers, and one under the TT.
The beams should go perpendicular to the joists, and can be held in place with jacks which allow more or less tension to level and dampen the floor. This serves to reduce the natural reasonant frequency of the floor thus reducing vibrational displacement. It is a relatively inexpensive approach and if you are not satisfied you can retreat and try something else.
I did this on a second floor living room over a garage, and it worked extremely well. You could dance on the floor without any affect to the TT. I would also recommend putting the TT as close to a wall over a foundation or load bearing wall as possible, as this is where the displacement is the least.
The beams should go perpendicular to the joists, and can be held in place with jacks which allow more or less tension to level and dampen the floor. This serves to reduce the natural reasonant frequency of the floor thus reducing vibrational displacement. It is a relatively inexpensive approach and if you are not satisfied you can retreat and try something else.
I did this on a second floor living room over a garage, and it worked extremely well. You could dance on the floor without any affect to the TT. I would also recommend putting the TT as close to a wall over a foundation or load bearing wall as possible, as this is where the displacement is the least.