after 3 years, I suspect it's not a shielding problem but a problem with one of the speakers or the internal bi-amp- if it has a single bi-amp that feeds both speakers, skip the next step
to rule out the source being the problem, you should switch leads from your mixer/soundcard, whatever your using to mix through, to make sure the problem IS with the monitor -.if that checks out OK... meaning the cracking stays with the monitoring regardless of whether its
try and determine if it's just coming from one speaker- if so, before running out and buying a new replacement speaker, swap speakers from the 'good' monitor. This may require a soldering iron, but may simply be 4 screws/nuts and a couple of plugs- IF the wires are not color coded, mark which wire goes to which terminal (polarity is important)
If the speaker is the culprit, look for a replacement, either from M-Audio, or one of the audio electronics outlets (Parts Express, etc)
If the speakers all seem to pass muster, then the bi-amp is likely the problem.... either take to the tech, or buy new monitors, unless you have enough electronics skills to trouble shoot circuit boards
Good luck!