Haven't posted here in a while.
I bought an MWF-15 with a hum a couple of years back. I did the recommended fix, and it didn't hum for about a month. Then the hum came back. I decided to use the outboard amp and pulled all the guts out. Used Atlantic Technology amp and hated it. So I decided to sell the sub. I had a perspective buyer who asked me to test the old amp to be sure it hums but works. So I connected the original amp, which was on the floor about 1.5 feet away from the back the sub to driver, turned it on, and - ZERO hum. The moment I installed it in - LOUD HUM.
Unscrew the amp and pull it out to see if it hums when pulled out. It might have just taken a little longer to warm up and show the issue. When you pull it out, check exactly where the cable running to the upper control panel routes, as this cable being too close to the transformer could cause a hum. If you didn't have the control panel connected, that could have been the difference as it cuts out a lot of the circuitry.
Seaton Sound, Inc. firstname.lastname@example.org 773-290-8436