String - you might check a couple of things like cones on speakers tight against washers, check also for resonance in room furniture, tables, art on wall ( ask me how I know ! Ugh ), also use your vandertones to see if it is isolated to sub
put heavy weight on external filters, try sweep tone disc to try and isolate ( Cardas)
... bummer
hope amp solves it... did they replace power supply caps ? When those are weak but not toast they can sound like woofer bottoming but normally also accompanied by a raise in 60 HZ noise
you might also pop rail fuses...