Well, this JBL ES250P sub is about 5 years old. Just today when I turned it on, it started making the hum noise. Amp still plays fine, but that hum is there. Unplugged line in, still hum. Tried another circuit plug in the house, still hum. I opened the amp, but I see no capacitors to replace that look bad.
If you unplug the power cord, does the hub disappear immediately or does it fade out? If it stops immediately, it might be the bridge rectifier or the transformer. If you are comfortable with measuring voltages on live equipment check that B+ and B- are equal and opposite. If not, measure the AC output of the transformer. If you don't understand this: DON"T TOUCH IT
It is probably the two main filter caps that are going bad. Those filter out the 60hz hum. In your pictures, they are the two biggest ones, marked SK(P). The value is printed on the other side. If you can provide that, a suitable replacement can be located. A quality replacement might be $5-6 for each cap, but worth it in my opinion.