D.M. Procida wrote:

Would speaker spikes help cut down the sound transmitted
to the
occupants of a flat on the floor below? If not, what
would?

I believe the floor is carpeted, and the speakers are not
very huge
anyway.

If the floor is resonating, some experimentation with layout
of furniture may help slightly, but it would have to be
really heavy to make much of a difference.

Otherwise your only hope is to persuade them to turn it down
or roll off the bass somehow. Your local council may help if
you get desperate, but it's a long haul that way.

A builder may be able to advise on installing beams, or a
pillar or two, to support the floor from below...

Ian