Woofers pumping

Well, the speaker is just a motor.  No reason B&W should know, if the upstream signal is causing it to move.  The woofer is just a slave to the signal.  Nothing to do with them. 

Can you explain a little more what you mean by pumping?  Does it shiver quickly?  Does it sloooowly move out and back?  Does it seem to be trying to make bass, but nothing comes out?

If it's solid state, this may happen with an intermittent regulator.  You may lose a + or - rail, but the coupling cap will eventually compensate for it.  Maybe. :D

If the pumping you describe was slow, and it’s a tube preamp you may have a leaky coupling cap. Needs to be replaced. More dangerous with DC coupled amps.

If the woofer pops in or out and slowly returns to a center position, it's likely another cap in the circuit. The coupling cap eventually charges to DC and blocks it.

OP:  You may have something going on upstream.  Tube pre?

Well, if it's playing music it's normal... :D

Depends on a lot of things, but if it does not happen when there is no music playing and if you swap the inputs to the amp it switches speaker then you know it's a signal issue.