As TomDac mentioned, try vacuuming them first. If that doesn't fix the issue, try checking the connections to the panel as khenegar1 suggested.
I have two suggestions...
1) I would open the back of the woofer enclosure to gain access to the power supply and crossovers. Any connections that are terminated via screws or spades, I would remove them, inspect them and reconnect them. It could be as simple as oxidation on the connections, and simply removing them and reinstalling them could fix the issue, making some fresh wire to metal contact.
2) Those Sequel II's are probably close to or over 30 years old. Any electrolytic caps that are in the power supply section could certainly stand to be replaced as those may be the culprit. Also, if there are any electrolytic caps in the high-pass section of the crossover (the panel section), those could also be a source of the problem and should be replaced.
Tosh, from the few various pictures I was able to pull up on Google the Sequel II crossovers look nothing like my SL3 crossovers, so doubtful that the SL3 schematic would be of any use, not to mention there were three different revisions of the SL3 crossover network over the years.