DIAGNOSIS Dust and skin oils can build up on the acoustically transparent fabric. You're right to be careful—harsh chemicals or rubbing can push dirt into the mesh or discolor it.
FIX The official and safest method is using a vacuum cleaner with a soft brush attachment. Gently run the brush over the grille to lift away surface dust. For smudges or spots, use a barely damp (not wet) microfiber cloth. Dampen the cloth with water, wring it out completely so it's just slightly cool to the touch, and gently dab—don't wipe hard—at the spot. Let it air dry completely before powering on.
IF THAT DIDN'T WORK For more persistent grime, you can use a very mild soap solution. One drop of dish soap in a cup of water. Dampen the microfiber cloth in the solution, wring it out until it's nearly dry, and dab gently. Immediately follow with a second microfiber cloth dampened with plain water (and well-wrung) to 'rinse' the area, then dry with a third clean, dry cloth.
HEADS UP Never spray any liquid directly onto the soundbar. Never use solvents, window cleaners, abrasives, or compressed air, as these can damage the fabric or drive moisture and particles into the drivers behind it. The grille is not designed to be removed by users on the 600 model.