Can the main filter be cleaned at all? Perhaps with another dust extractor just to pull the visible soiling off the surface of the flutes of the filter?

Best practice for a clogged filter is changeover as the dust can inpregnate into the filter folds and clog once again very quickly and reduce performance on the tool.

The L & M class filters can be cleaned by extracting dust with a suitable dust extractor of the dust class being removed, however we couldn't recommend it for H class filters that are used with hazardous dust.