<font color='#000000'>Doppler distortion in loudspeakers is generally a myth. Time permitting I may write something on this in the future.
Many loudspeakers which offer the mass loading feature are usually not braced well enough and require extra stability to dampen cabinet resonances. This may be a good compromise to cut down production costs and let the customer provide the added stability to their liking should they find it necessary.
My recommendation is to at least try it assuming your speakers have that feature.
As for speaker stands, I always recommend mass loading them, especially if they are metal, to prevent unwanted resonances or ringing. Dry sand is usually the most economical choice, just try not to spill any on the floor
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