Electrostatic loudspeakers are getting older during the years. They are contineously changing their physical state. Mechanical tension and diaphragm conductivity are sensible factors to reproduction quality.
This fact is not that much important for fulltange ESLs, because the whole frequency range is covered by one sindle driver. Hybrid ESLs are different, as the ESL tweeter changes relatively to the bass/mid driver and looses its original acoustical balance.