I'd suggest checking them out individually. It doesn't make logical sense that both speakers would display the same anomaly if the problem was with a driver, drivers or crossover.
If they both respond the same during the individual troubleshooting, then my money is on a problem upstream.
Do you have another pair of full range speakers available, or even a pair of monitor speaker that go into to 50Hz range, to install to rule out an upstream problem?
If they both respond the same during the individual troubleshooting, then my money is on a problem upstream.
Do you have another pair of full range speakers available, or even a pair of monitor speaker that go into to 50Hz range, to install to rule out an upstream problem?

