Product review (5 stars): High spin, no downstairs rumble...
I figured these had the best reviews and with Amazon no hassle return for failure to complete the task why not.  In short, installed on washer and dryer, both front loading.  Washer is like a freight train in high spin, yes it is level, but it is on the second floor and house was built to where the washer sits in floor attached emergency drain pan (made for washers).  So making floor stiffer is not possible unless I lift the pan... etc.. nightmare.  So after 8 years of hearing things rattle on the walls I decided to go ahead and give this a go.  Why did I wait???  It took around 30 minutes with a single person and cleaning under the washer and dryer while they were tilted.  I then placed the washer on a high speed heavy duty cycle with a full load, (Samsung front loader if it matters).  I can feel the washer moving in spin, but vibration as advertised is almost non-existent when transferring this motion to the floor.  Sometimes the simplest things shock you as a resolve, it couldn't be that simple... oh yes it could.  Hopefully it works for others as well.