Woofers pumping

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

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.

@audionoobie LOL!

@erik_squires No. Topping Pre90

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

@erik_squires Thank you for the info. I'll contact Topping and see if I can get any info. What really surprised me was the tech at B&W didn't have any idea what could cause it to happen!!!