Well, even if you change the mat to a different type, the tonearm hasnt been isolated further, so, it could be picked up that way. Remember, the phono cartridge and stylus are designed to pick up extremely small vibrations and signals, so, what Im saying could be possible.
Still, what you suggest also makes sense, and indicates the Numark and Stanton tables to not be as well designed as they should be? Could be some electrical components to close to sensitive signal wires, not enough shielding around the transformer, poor grounding.
This is definitely an issue I have never had with the Technics 1200,s.
Mr. Scott Fitlin