I have a MIT Z Stabilizer now, got it used as a gift. A double blind test revealed no change when used with my audio equipment. HOWEVER, it did stop a digital clock in the house from running fast, with the MIT Z Stabilizer plugged into the same circuit the clock is, the clock now keeps perfect time. I did some measurements on the thing and what the device is (at least the "older" ones described here) are series resonant filters connected across the power line set to harmonics of the power line, i.e. 120, 180, 240, 360 Hz. The switch adds a couple more filters for even higher frequency harmonics.
The clock was probably picking up extra counts from the harmonics on the power line. The device could be helpful for some CD players or other digital sources with inferior power supplies that suffer degraded performance with noisy power. It also might help computers that suffer from random reboots for no known reason. But for analog audio equipment with properly designed power supplies, I can't see it making any difference, at least it made no difference with my equipment.
