I did a bit of Googling and here's what I came across:

2. Dusting your piano

Dust is very abrasive, and can scratch the finish if wiped off with a dry cloth. To avoid scratching, dust the piano lightly with a feather duster. Alternatively, wipe lightly with a soft damp cloth to pick up the dust, followed immediately with a dry cloth. The cloths should be soft cotton such as flannel, because coarse or synthetic fabrics can scratch some finishes. Wring out the damp cloth thoroughly so it leaves no visible moisture on the surface.

To avoid creating swirl marks, always wipe with long straight strokes rather than circular motions. Wipe with the grain for natural wood finishes, or in the direction of the existing sheen pattern for solid-color satin finishes.
http://www.ptg.org/resources-pianoOwners-finishCare.php

I'm guessing that this is pretty similar for speakers with a piano black finish. Talk about high maintenance! With that said, I wonder if you can get a piano technician to fix up the finish of your piano black speakers.