Sent email to Whirlpool about the vibration issue, here's their
response -

===========================

From:

Whirlpool Horizontal Axis washer model GHW9400P has a maximum spin
speed of 1200 RPM, and model GHW9150P has a 1000 RPM spin speed.

Due to the high spin speeds that the Duet washer can achieve, a floor
reinforcement may be necessary to reduce vibrations if the pair is
installed on a second floor or over a basement. We recommend using
3/4 inch plywood the size of the bottom of the unit to be screwed
every 6 inches with 3 inch drywall screws.

The Horizontal Axis is a term used by Whirlpool because of the way the
tub and drive motor system are placed in the cabinet. It has a
world-class 6-point suspension system that features dual springs to
isolate vibration and four shock absorbers at the washer base to
minimize movement. If it detects any unbalance, the washer
automatically attempts to redistribute the load up to seven times.
Even the largest loads spinning at high RPMs can remain in complete
balance.

When installing on a second floor, you will need to make sure that the
floor is able to support 600 pounds for the large Duet pair or 400
pounds for the Duet Sport.

The Duet Sport model WFW8300S washer and WED8300S electric dryer,
WGD8300S gas dryer are new and manufactured with 2nd floor
installation in mind. The Duet Sport washer features four shock
absorbers and two support springs that cushion the motion of the inner
drum as it spins, reducing vibration and noise.


--

I've wrestled with reality for 35 years,
and I'm happy to state I finally won out over it.

....Mary Chase