Some of the current Fender amps use black plastic jacks. I've seen several amps where the nut holding the input jacks to the chassis has loosened and the jack is loose. These jacks are mounted on the PC board, and on all the amps I've seen, the solder traces have been broken. I'm guessing this is because the owner doesn't pay any attention to a loose jack, and only deals with it when the amp starts cutting out or buzzing because of a broken connection.
It's possible to replace these jacks with chassis mounted ones and run shielded wire to the board. However if you use metal Switchcraft-type jacks you need to be aware that, depending on the amp, you might be setting up a
ground loop. On the Blues Deville you'll know this has happened if you turn on the Drive channel, and start to turn up the Drive and Master volume. At some point the amp will squeal, even without anything plugged in.The reason is that the metal jack now adds a second path to ground through the chassis. Previously the plastic jack was grounded only through a connection to the circuit board.
I've dealt with this by using insulating washers that line the hole and insulate the jack and mounting nut from the chassis. Another way would be to use a non-conductive jack. Nylon jacks are available from Cliff or Reans, and these would probably work as well.