Those hum noises are pretty much always created by ground loops. Noise can be generated when interconnected components don’t have the same ground resistance because the electricity will follow the path of least resistance, which may not be through the ground point to which it’s directly connected. If that’s the case, the electricity will flow through the ground wires of the signal cables to reach the least resistive path back to ground. This can induce voltage in the signal cables that is the source of the hum. Unbalanced cables are much more likely to cause hum because the signal negative connection is the same connection as the electrical ground. Balanced cables are better because they have a ground connection that is independent of the signal negative. 

It is generally the case that the signal grounds back at the source, which is whatever is at the other end of the XLR cable feeding signal to the subwoofer, as well as at the subwoofer/speaker (receiving) end. The subwoofer amps reference the same ground for both power and signal. When the subwoofer is connected to a mixer, there will be 2 ground points, 1 at the subwoofer’s ground pin and the other at the mixer’s. This can create the infamous ground loop. If there is a sufficient difference in resistance between the respective ground paths, which is to say there is a different degree of resistance to ground through the power-plug ground pin of the subwoofer compared to the power-plug ground pin of the mixer, this can create a ground loop. And if the mixer doesn’t have its own ground, then it may ground through the signal cable to the subwoofer(s). The best way to check for this condition is to connect both components to the same outlet. If they are connected to the same outlet, then they are connected to the same ground point and there should be no hum. 

One thing that you should verify is that all your outlets are wired correctly. This can be done with a simple accessory such as one of these types: 

Commercial Electric Outlet Tester

or

Receptacle-Electrical-Klein-Tools

Verify that you’re using balanced cables between the mixer and speakers. Also verify that you signal cables are wired correctly.

OSP-CT-04C-Cable-Tester-Speakon

The next step is to connect the subwoofer and mixer to the same ground, which is done by connecting all the components to the same outlet, if necessary using a grounded power strip.  If there’s no noise in that configuration, then move the mixer to another circuit. If there’s noise, you should check the ground on the new circuit. If it doesn’t make noise with the mixer connected to a different circuit, the noise may be being introduced by components that are connected to the mixer. Connect each component to the mixer one at a time to determine which part is introducing the loop. Seek to ground the offending part. (Especially if it’s connected via unbalanced signal cables.) 

If there is a hum present when only the mixer and a subwoofer are connected to the same outlet then there is a problem with one of those two components. You then need to switch out the subwoofer for another one to see if it still happens, and then for a different speaker to see if it still happens. If the noise is present with one subwoofer but not the other, then the subwoofer with the hum needs service. If the noise is present with both subwoofers but not with any other components, then both subwoofers may need service. If the hum is present with any other components independently from the subwoofers then the mixer may need service. If the hum is only present when a specific combination of components are connected, there may be an incompatibility problem. Some older components use different pin assignments, so special adapter cables are necessary.