I have had the Spatial M4s since the end of May 2016 and there is no accumulation of dust that I can detect in the open woofers as of now. This time period included a total remodel of the kitchen so that doesn't seem to be a problem. They are actually somewhat protected by the slanted baffle so a little careful dusting (vacuum) from time to time should be fine.
As far as picking up hum, I have only tried these with some classic amps, a New York Audio Labs moscode 300, a Marantz 140 amp, a PSE Studio 2, some modded low output tube amps, and the Anthem i225 integrated which has a beautiful dark background. No hum that I could notice unless standing with my ear next to the compression driver and the NYAL moscode 300. They are very efficient though so it is possible some hum would come through certain components.
As far as picking up hum, I have only tried these with some classic amps, a New York Audio Labs moscode 300, a Marantz 140 amp, a PSE Studio 2, some modded low output tube amps, and the Anthem i225 integrated which has a beautiful dark background. No hum that I could notice unless standing with my ear next to the compression driver and the NYAL moscode 300. They are very efficient though so it is possible some hum would come through certain components.