Very sorry for the late reply.... I really have to check in here more often!
These select the connection of ground to from the input (source) side to the output (amp) side of the pre-amp.
'F' is Float mode - no connection from input to output
'H' Is Hard Ground mode - the input ground is connected to the output ground.
Which to use is totally system dependant - there is no right or wrong or better way to set it. If your system is quiet then that is the right setting. Some systems do not care at all and are quiet in either mode. Some are a bit quieter in one or the other and once and a while a system really needs to be set one way or another.