Over time, the amount of RF noise in the air that the electronics are subject to has grown. Thus, to keep the electronics quiet, the circuitry has been modified over the years. The older the instrument is, the more susceptible it is to this RF noise; but it depends on the instrument, and it depends on the environment. Alembic offers an electronics upgrade for older Series instruments that addresses this noise issue. The upgrade is not inexpensive. However, most older instruments have not had the upgrade and their owners seem very happy with them. As far as the tone shaping circuitry, the electronics are essentially unchanged over time. My '86 S1 is a little susceptible to RF noise, but I never thought it to be a problem; and if I hadn't switched to six string, it would still be my primary instrument.