"You can only do this if the preamp employs a 2-prong AC connection! "
Actually, can be done with a three prong connection as well....just have to switch the live and neutral within a plug of an ac power cord, and resupply the "noisy" preamp or amp ...in some cases the noise will be less........lifting the ground is way to break the ground loop, but is fraught with electrical shock hazard, although it can be a useful tool or trick to determine the origin of a ground loop...some preamplifiers - eg the Herron VTSP series have polarity switching toggle switch at the back to enable this to reduce "noise" in the system.
Actually, can be done with a three prong connection as well....just have to switch the live and neutral within a plug of an ac power cord, and resupply the "noisy" preamp or amp ...in some cases the noise will be less........lifting the ground is way to break the ground loop, but is fraught with electrical shock hazard, although it can be a useful tool or trick to determine the origin of a ground loop...some preamplifiers - eg the Herron VTSP series have polarity switching toggle switch at the back to enable this to reduce "noise" in the system.